l?po 

Columbia  Statoertfitp 

intljeCttpofBfttigork 

College  of  $fjps:tctanjs  ano  burgeons! 
Hibvatp 


vO 


Tl 


f^se 


%i* 


ri\ 


i 


1 


ul 


ix- 


Digitized  by  the  Internet  Archive 

in  2010  with  funding  from 
Columbia  University  Libraries 


http://www.archive.org/details/lessonsinelemenOOhuxl 


LESSONS 


ELEMENTARY    PHYSIOLOGY 


•?&&&■ 


LESSONS 

IN 

ELEMENTARY  PHYSIOLOGY 

BY 

THOMAS   H.   HUXLEY,   LL.D.,  F.R.S. 


EDITED 
FOR  THE  USE  OF  AMERICAN  SCHOOLS  AND  COLLEGES  BY 

FREDERIC   S.   LEE,   Ph.D. 

Adjunct  Professor  of  Physiology  in  Columbia  University 


WITH  NUMEROUS  ILLUSTRATIONS 


THE    MACMILLAN    COMPANY 

LONDON:   MACMILLAN  &  CO.,  Ltd. 

1916 

AH  rights  reserved 


Copyright,  1900, 
By  THE  MACMILLAN  COMPANY. 


Sit  up  and  electrotyped  February,  1900.     Reprinted  October, 
1902;  April,  1903;  October,  1904  ;  October,  1905;  October,  1906: 
July,  1907;  July,  1909  ;  September,  1910;  January,  1912; 
September,  1913;  October,  1914;   March,  1916. 


QP36 


NotrjjanU  $K00 

J.  S.  Cushing  &  Co.  —  Berwick  &  Smith 
Norwood  Maas   U.S.A. 


EXPLANATION   OF   THE    PLATE 


The  Human.  Skeleton  in  Profile 


Na. 
Fr. 

Pa. 

Oc. 

Mn. 

St. 

R. 

R>. 

S. 

Cx. 

Sep. 

CI. 

H. 

Ra. 

U. 

CP. 

Mc. 

D. 

I,  If, 

II. 

Pb. 

Is. 

F. 

Tb. 

Fb. 

T. 

Mt. 

D. 


The  Nasal  bones. 

The  Frontal  bone. 

The  Parietal  bones.  \  In  the  Skull. 

The  Occipital  bone. 

The  Mandible,  or  Lower  Jaw. 

The  Sternum,  or  Breast-bone. 

The  Ribs.  In  the  Thorax. 

The  Cartilages  of  the  Ribs. 

The  Sacrum. 

The  Coccyx. 

The  Scapula,  or  Shoulder-blade. 

The  Clavicle,  or  Collar-bone. 

The  Humerus. 

The  Radius. 

The  Ulna. 

The  Carpus,  or  Wrist-bones, 

The  Metacarpus. 

The  Phalanges  of  the  Fingers,  or  Digits  of  the 

Hand. 
Ill,  IV,  V.     The  Pollex,  or  Thumb,  and  the  succeeding  Fingers. 

The  mum.      j  h         ther  form  the  Hip-bone, 

The  Pubis. 

The  Ischium. 

The  Femur. 

The  Tibia. 

The  Fibula.  I-  In  the  Leg. 

The  Tarsus,  or  Ankle-bones. 

The  Metatarsus. 

The  Phalanges  of  the  Toes,  or  Digits  of  the  Foot.  J 


In  the  Arm. 


or  Os  innominatum. 


PREFACE 

Huxley's  "  Lessons  in  Elementary  Physiology  "  was  pub- 
lished first  in  1866.  The  last  edition  which  the  author 
himself  brought  out,  was  the  revised  edition  of  1885.  The 
book  has  recently  undergone  an  extensive  and  careful 
revision  at  the  hands  of  Professor  Michael  Foster  and  Dr. 
Sheridan  Lea.  In  the  preface  to  this  English  edition 
Professor  Foster  writes  as  follows  :  — 

"  My  friend,  Dr.  [Sheridan]  Lea,  and  I  have  undertaken 
a  task  of  great  difficulty. 

"The  progress  which  has  taken  place  in  science  and  in 
education,  since  the  last  revision  of  this  work,  has  rendered 
it  desirable  to  make  considerable  changes  and  additions  in 
order  to  maintain  for  the  book  the  usefulness  which  has 
been  for  so  long  a  time  its  conspicuous  feature. 

"  At  the  same  time  a  pious  feeling  has  led  us  to  preserve 
as  far  as  possible  the  original  author's  own  form  of  exposi- 
tion and,  indeed,  his  own  words.  We  have  done  our  best 
to  secure  both  of  these  ends. 

"  Although  I  share  with  Dr.  Lea  the  responsibility  of  all 
the  changes  which  have  been  made,  the  main  labour  has 
fallen  on  him  ;  and  I  may  say  that  both  his  larger  and  my 


vi  PREFACE 

smaller  share  in  the  work  have  been  truly  labours  of  love, 
small  tributes  of  affection  to  the  master  who  is  no  more." 

In  the  preparation  of  the  present  edition,  which  the  pub- 
lishers desired  for  use  in  America,  it  has  been  deemed 
advisable  to  make  many  alterations  of  the  latest  English 
text,  in  order  to  adapt  the  book  to  the  needs  of  those  classes 
of  American  students  who  most  demand  it.  The  present 
writer  has  performed  his  task  with  a  long-standing  feeling 
of  affection  for  the  pages  which  introduced  him  to  the 
study  of  Physiology,  and  first  gave  him  a  clear  insight  into 
the  nature  of  scientific  conceptions  and  scientific  reasoning. 


FREDERIC  S.  LEE. 


Columbia  University,  New  York, 
January,  190x3. 


CONTENTS 


LESSON    I 

A  GENERAL  VIEW   OF  THE   STRUCTURE   AND   FUNC- 
TIONS  OF  THE   HUMAN   BODY 

PAGE 
SEC. 

I 


The  Work  of  the  Body  as  a  Whole 

10 


2.  The  General  Build  of  the  Body 


2.  The  Tissues 

4.  The  Skeleton 

C.   The  Erect  Position I? 

20 

6.  Sensory  Organs   .         .         •         •• 

7.  The  Renewal  of  the  Tissues 2I 

8.  Alimentary  Organs 

9.  Circulatory  Organs 

10.  Excretory  Organs 23 

11.  Respiratory  Organs 3 

12.  Coordinating  Action  of  the  Nervous  System  ...       25 

13.  Life  and  Death 

(i)  Local  Death 2° 

26 

27 

2$ 


u- 


(ii)  General  Death 

Modes  of  Death 27 


15.  Decomposition  of  the  Body 

LESSON    II 

THE   MINUTE   STRUCTURE   OF  THE  TISSUES 


Every  Tissue  a  Compound  Structure 3° 

The  Embryonic  Tissues  and  the  Cells.     Protoplasm      .         •       31 
The   Body  starts   as  a   Single   <  'ell,  the   Ovum,  which   then 

divides  into  Primitive  Cells 32 

Tii 


CONTENTS 


4.  The  Differentiation  of  the  Primitive  Cells 

5.  The  Chief  Tissues  of  the  Body     • 

6.  The  Structure  of  the  Epidermis   . 

7.  The  Growth  of  the  Epidermis 

8.  The  Unit  used  in  Histological  Measurement 

9.  The  Epithelium  of  Mucous  Membrane 

10.  The  Structure  of  Cartilage  . 

(i)   Hyaline  Cartilage    . 
(ii)   White  Fibro-cartilage 
(iii)   Yellow  or  Elastic  Fibro-cartilage 

11.  The  Development  of  Cartilage 

12.  The  Structure  of  the  Connective  Tissues 

(i)  Areolar  Tissue 
(ii)  Other  Varieties  of  Connective  Tissue 

13.  The  Development  of  Connective  Tissue 


PAGE 

33 

34 
35 
38 
40 

4i 

42 
42 
46 
46 
47 
49 
49 
5i 
53 


LESSON    III 


THE  VASCULAR   SYSTEM   AND  THE  CIRCULATION 


Part  I.     The  Blood  Vascular  System  and  the 
Circulation  of  Blood 

1.  The  Capillaries    .         .        ' 55 

2.  The  Arteries  and  Veins 56 

(i)  The  Structure  of  an  Artery        .  .  .  .  -57 

(ii)  The  Structure  of  a  Vein    ......  59 

3.  The  General  Arrangement  of  Blood-vessels  in  the  Body         .  61 

4.  The  Heart  ..........  66 

5.  The  Valves  of  the  Heart      .......  69 

6.  The  Structure  of  the  Heart           ......  74 

7.  The  Beat  of  the  Heart 75 

8.  The  Action  of  the  Valves 76 

9.  The  Working  of  the  Arteries         ......  80 

10.  The  Cardiac  Impulse  ........  81 

11.  The  Sounds  of  the  Heart 82 

12.  Blood-pressure     .         .         .         .         .          .         .    '      .          .  83 

13.  The  Pulse .85 


CONTENTS  u 

SEC.  PAGE 

14.  The  Rate  of  Blood  Flow       .         .         .  '       .         .         .         .89 

15.  The  Nervous  Control  of  the  Arteries.     Vaso-motor  Nerves    .       90 

16.  The  Vaso-motor  Centre 94 

17.  Vaso-dilator  Nerves      .  .  .         .  .  .  .96 

18.  The  Nervous  Control  of  the  Heart.     Cardiac  Nerves    .         .       98 

19.  The  Proofs  of  the  Circulation 103 

20.  The  Capillary  Circulation 105 

21.  Inflammation       .........      107 

Part  II.     The  Lymphatic  System  and  the  Circula- 
tion of  Lymph 


1.  The  General  Arrangement  of  the  Lymphatics 

2.  The  Oiigin  and  Structure  of  Lymphatics 

3.  The  Structure  and  Function  of  Lymphatic  Glands 

4.  Causes  which  lead  to  the  Movements  of  Lymph    . 


109 
112 

"5 
117 


LESSON    IV 


THE   BLOOD   AND  THE   LYMPH 


1 .  Microscopic  Examination  of  Blood 

2.  The  Red  Corpuscles    . 

3.  The  White  Corpuscles 

4.  Blood  Platelets    .... 

5.  The  Origin  and  Fate  of  the  Corpuscles 

6.  The  Physical  Qualities  of  Blood  . 

7.  The  General  Composition  of  Blood 

8.  The  Proteids  of  Plasma 

9.  The  Clotting  of  Blood 

10.  The  Quantity  and  Distribution  of  Blood  in  the  Body 

11.  The  Functions  of  the  Blood 

12.  Lymph:  its  Character  and  Composition 

13.  The  Mode  of  Formation  of  Lymph 

14.  The  Functions  of  the  Lymph 


119 
122 
126 
130 
130 

131 
132 

134 
136 
140 
141 
142 
144 
147 


CONTENTS 


LESSON  V 


RESPIRATION 

SEC. 

1.  The  Gases  of  Arterial  and  Venous  Blood 

2.  The  Nature  and  Essence  of  Respiration 

3.  The  Organs  of  Respiration  . 

4.  The  Thorax  and  Pleura 

5.  The  Movements  of  Respiration 

6.  The  Amount  of  Air  respired 

7.  The  Changes  of  Air  in  Respiration 

8.  The  Amount  of  Waste  which  leaves  the  Lungs 

9.  The  Nature  of  the  Respiratory  Changes  in  the  Lung: 

Tissues         .         .         . 

10.  The  Nervous  Mechanism  ot  Respiration 

11.  Influence  of  Blood-supply  on  the  Respiratory  Centre 

pncea  and  Asphyxia      ..... 

12.  The  Influence  of  Respiration  on  the  Circulation   . 

13.  Ventilation  ....... 


s  and 


Dys- 


152 

J54 
160 
162 
172 
174 
175 

177 
179 


191 


LESSON   VI 


THE   SOURCES   OF   LOSS   AND  OF  GAIN  TO 
THE   BLOOD 

1.  General  Review  of  the  Gain  and  Loss 

2.  Secretion  in  General    . 

3.  The  Urinary  Organs     . 

4.  The  Structure  of  a  Kidney 

5.  The  Urine  . 

6.  The  Secretion  of  Urine 

7.  The  History  of  Urea    . 

8.  The  Structure  of  the  Skin.     Nails  and  Hairs 

9.  The  Composition  and  Quantity  of  Sweat 

10.  The  Secretion  of  Sweat  and  its  Nervous  Control 

11.  A  Comparison  of  the  Lungs,  Kidneys,  and  Skin 

12.  Animal  Meat:   its  Production  and  Distribution 

13.  Regulation  of  Body-temperature  by  Altered  Loss  of  Heat 


193 
199 
2b  1 
203 
208 
211 
213 
215 
223 
224 
226 
227 
229 


CONTENTS 


14.  Regulation   of  Body-temperature   by  Altered   Production   of 

Heat   ....         ...         ...  232 

15.  The  Temperature  of  Fever  .  .  .  .  ...  .  233 

16.  The  Structure  of  the  Liver  .......  233 

1 7.  The  Work  of  the  Liver         .  239 

18.  The  Spleen  .........  244 

19.  The  Thymus  Gland      ........  246 

20.  The  Thyroid  Body  or  Gland  ......  246 

21.  The  Suprarenal  Bodies         .......  247 


LESSON   VII 


THE  SOURCES   OF  LOSS  AND  OF  GAIN  TO  THE 

BLOOD  {Continued):     THE   FUNCTION 

OF  ALIMENTATION 

Part  I.     Digestion  and  Absorption 

1.  Waste  made  Good  by  Food  ......     249 

2.  Food  and  Food-stuffs  ........     250 

A.  Nitrogenous  Food-stuffs      ......     250 

B.  Non-nitrogenous  Food-stuffs       .         .         .         .         .251 

(i)  Fats 251 

(ii)   Carbohydrates  .  .  .  .  .  ,251 

(iii)   Salts  and  Water        .  .  .  .  .251 

3.  The  Purpose  and  Means  of  Digestion  .....     252 

4.  The  Mouth  and  the  Teeth   .......     252 

5.  The  Development  of  the  Teeth    .         .         .         .         .         .257 

6.  Mastication  .........     260 

7.  The  GEsophagus  and  Swallowing  .  .         .  .  .261 

8.  The  Salivary  Glands     ........      262 

9.  Saliva  and  its  Secretion        .......     265 

10.  The  Action  of  Saliva    ........     267 

11.  Soluble  Ferments  or  Enzymes       ......      267 

12.  The  Structure  of  the  Stomach       ......      268 

13.  Gastric  Juice  and  its  Secretion      ......     270 

14.  The  Action  of  Gastric  Juice  .  .  .  .  .  .271 

15.  The  General  Arrangement  and  Structure  of  the  Intestines     .     274 


CONTENTS 


SEC.  PAGE 

1 6.  The  Structure  of  the  Villi     .......  280 

17.  Succus  Entericus          ........  282 

18.  The  Structure  of  the  Pancreas  and  its  Changes  during  Secretion  282 

19.  The  Nature  and  Action  of  Pancreatic  Juice  ....  283 

20.  The  Function  of  Bile  ........  285 

21.  The  Changes  undergone  by  Food  in  the  Intestines         .         .  285 

22.  Absorption  from  the  Intestines     ......  288 

Part  II.    Food  and  Nutrition 

1.  Some  Aspects  of  Nutrition  .         .         .         .         .         .         .291 

2.  Some  Statistics  of  Nutrition  .         .         .         .         .         .292 

3-  Diet 295 

4.  The  Economy  of  a  Mixed  Diet     .         .         .         .         .  297 

5.  The  Effects  of  the  Several  Food-stuffs  .....  300 

6.  The  Erroneous  Division  of  Food-stuffs  into  Heat  producers 

and  Tissue-formers        .         .          .....  302 

7.  The  Income  and  Expenditure  of  Energy       .  303 


LESSON    VIII 


MOTION  AND   LOCOMOTION 

1.  The  Source  of  Active  Power  and  the  Organs  of  Motion 

2.  Ciliated  Epithelium  and  the  Action  of  Cilia 

3.  The  Structure  of  Unstriated  Muscle 

4.  The  Structure  of  Striated  Muscle 

5.  The  Chemistry  of  Muscle     . 

6.  The  Phenomena  of  Muscular  Contraction 

7.  The  Tetanic  Contraction  of  Muscles     . 

8.  The  Various  Kinds  of  Muscles     . 

Muscles  not  attached  to  Solid  Levers 
Muscles  attached  to  Definite  Levers 

9.  The  Structure  of  Bone 

10.  The  Development  of  Bone  . 

11.  The  Mechanics  of  Motion.      Levers 

1 2.  The  Joints  of  the  Body 

13.  The  Various  Movements  of  the  Body    . 


307 
308 
310 

3" 

3i8 
320 
323 
324 
324 
325 
328 
335 
34i 
345 
353 


CONTENTS  xiii 

SEC.  PAGE 

14.  The  Mechanics  of  Locomotion     ......  354 

15.  The  Mechanism  of  the  Larynx     ......  356 

16.  The  Voice  ..........  361 

17.  Speech        ..........  363 


LESSON    IX 


SENSATIONS  AND   SENSORY   ORGANS 


Movement  the  Result  of  Reflex  Action 
Sensations  and  Consciousness       .... 

The  Special  Senses       ...... 

The  General  Plan  of  a  Sense-organ 

The  Skin  as  a  Sense-organ  .... 

(i)  The  Sensation  of  Pressure 
(ii)  The  Sensations  of  Temperature 
(iii)  The  Sensation  of  Pain     .... 

(iv)  The  Localisation  of  Tactile  Sensations     . 
The  Muscular  Sense     ...... 

The  Sense  of  Taste      ...... 

The  Sense  of  Smell      ...... 

The  Ear  and  the  Sense  of  Hearing  in  General 
The  Membranous  Labyrinth  .... 

(i)  The  Utricle,  the  Saccule,  and  the  Membranous 
circular  Canals     ..... 

(ii)  The  Membranous  Cochlea 
(iii)  The  Organ  of  Corti  .... 

The  Bony  Labyrinth    ...... 

The  Middle  Ear 

(i)  The  Auditory  Ossicles      .... 

(ii)  The  Muscles  of  the  Tympanum 
The  External  Ear         ...... 

The  Transmission  of  Sound  Waves  to  the  Inner  Ear 
The  Conversion  of   Sonorous  Vibrations  into  Sensations   o 
Sound  ....... 

The  Mode  of  Action  of  the  Auditory  End-organs 
Localisation  of  Sound  ..... 


Semi 


367 
369 
37° 
371 
373 
378 
378 
380 
38i 
382 

383 

387 
392 
395 

395 
399 
402 
405 
406 
407 
409 
410 
410 

414 

4i5 
419 


xiv  CONTENTS 

SEC  PAGE 

1 8.  The  Functions  of  the  Tympanic  Muscles  and  Eustachian  Tube     420 

19.  The  Functions  of  the  Semicircular  Canals,  the  Utricle,  and 

the  Saccule  .  '    .         .         .         .         .         .     421 


LESSON   X 
THE  ORGAN   OF   SIGHT 

1.  The  General  Structure  of  the  Eye 423 

2.  The  Eye  as  a  Water  Camera         ......  428 

3.  The  Mechanism  of  Accommodation     .....  433 

4.  The  Limits  of  Accommodation.      Use  of  Spectacles      .          .  437 

5.  The  Muscles  of  the  Eyeball 439 

6.  The  Protective  Appendages  of  the  Eye          ....  440 

7.  The  Structure  of  the  Retina  .  .  .  .  .  -441 

8.  The  Sensation  of  Light         .......  448 

9.  The  "  Blind  Spot  " 449 

10.  The  Duration  of  a  Luminous  Impression       ....     450 

11.  Sensations  of  Light  produced  without  the  Action  of  Light     .     451 

12.  The  Functions  of  the  Rods  and  Cones  .         .         .         .         .452 

13.  Sensations  of  Colour  and  Colour-blindness    ....     453 

Colour-blindness      .         .         .         .         .         .         .         -457 


LESSON   XI 

THE  COALESCENCE  OF  SENSATIONS  WITH  ONE 

ANOTHER  AND   WITH   OTHER    STATES 

OF   CONSCIOUSNESS 

1.  Sensations  may  be  Simple  or  Composite 

2.  Judgments,  not  Sensations,  are  Delusive 

3.  Subjective  Sensations  .... 

4.  Delusions  of  Judgment 
t;.  The  Inversion  of  the  Visual  Image 
(>.  Every  Image  referred  U>  an  Object 
7.  The   judgment  of  Distance  and  Size  by  the  Brightness  an 

Size  of  Visual  Images  ...... 


459 
462 

463 
465 
466 

467 


CONTENTS 


8.  The  Judgment  of  Form  by  Shadows     .         .         .         .         .470 

9.  The  Judgment  of  Changes  of  Form  .....  471 
[O.  Single  Vision  with  Two  Eyes.  Corresponding  Points  .  .  472 
ti.  The  Judgment  of  Solidity    .......     473 


LESSON   XII 
THE  NERVOUS   SYSTEM   AND   INNERVATION 


1.  The  General  Arrangement  of  the  Nervous  System 

2.  The  Investing  Membranes  of  the  Cerebro-spinal  System 

3.  The  Anatomy  of  the  Spinal  Cord  and  the  Roots  of  the  Spinal 

Nerves         ........ 

4.  The  Structural  Elements  of  Nervous  Tissue  . 

5.  The  Structure  of  Nerves       ...... 


6.  The  Minute  Structure  of  the  Spinal  Cord  and  Spinal  Ganglia  490 

The  Cells  of  the  Grey  Matter.  .         .         .  .         .  491 

The  Differences  in  -Structure  of  the  Spinal  Cord  at  Various 

Levels  .........  492 

The  Structure  of  a  Spinal  Ganglion  .....  494 

7.  The  Functions  of  the  Roots  of  the  Spinal  Nerves.  .  .  495 

8.  The  Physiological  Properties  of  a  Nerve        ....  499 

The  Electrical  Properties  of  a  Nerve        .         .        ..         .  502 

The  Rate  of  Transmission  of  a  Nervous  Impulse       .  .  503 

9.  The  Functions  of  the  Spinal  Cord         .....  506 

Reflex  Action  through  the  Spinal  Cord     ....  506 

The  Paths  of  Conduction  of  Impulses  along  the  Spinal  Cord  5 1 1 


10.  The  Sympathetic  Nervous  System 

11.  The  Anatomy  of  the  Brain    . 

The  Corpora  Quadrigemina 

The  Optic  Thalami . 

The  Corpora  Striata 

The  Membranes  of  the  Brain   . 

12.  The  Minute  Structure  of  the  Brain 

The  Cerebellum 
The  Cerebral  Cortex 
I  3.  The  Cranial  Nerves 


475 
476 

477 
481 

483 


5H 
5*7 
526 
526 
528 
528 
529 
530 
532 
535 


xvi  CONTENTS 

SEC.  PAGE 

14.  The  Functions  of  the  Spinal  Bulb  or  Medulla  Oblongata        .  538 

15.  The  Functions  of  the  Cerebellum          .....  540 
.16.  The  Functions  of  the  Cerebral  Hemispheres          .         .         .  542 

The  Hemispheres  the  Seat  of  Intelligence  and  Will .         .  542 

Reflex  Actions  of  the  Brain     ......  545 

Localisation  of  Function  in  the  Cortex   of  the  Cerebral 

Hemispheres        ........  547 

17.  The  Paths  of  Conduction  of  Impulses  in  the  Brain        .         .  551 

APPENDIX 

ANATOMICAL  AND  PHYSIOLOGICAL  CONSTANTS 

I.  General  Statistics     ........  555 

II.  Nutrition 557 

III.  Circulation       .........  557 

IV.  Respiration 558 

V.  Cutaneous  Excretion        .......  559 

VI.    Renal  Excretion 560 

VII.  Nervous  Action 560 

VIII.  Histology 560 


LESSONS 

IN 

ELEMENTARY   PHYSIOLOGY 


<  <hs 


r  s 


H 

r/> 

j. 

fcs) 

> 

c/j 

<1 

U 

H 

C/3 

§ 

w 

r^ 

- 

H 

O 

H 

:/5 

£ 

H 

£ 

u> 

O 

£ 

<! 

H 

Qi 

w 

u 

^ 

u. 

o 

& 

fc 

■« 

O 

•5 

v. 

k 

<* 

<: 

g 

a. 

<5 

S 

n 

u 

K 

.s?  s 


o    m   o    v 


~    :      ~     _ 


tv.     3    ■"■     CO    '""' 


O       <^   M3         •     'vD 


£      .*  .2  j? 


e 

bo    > 

,_^    c 

* 

o 

B    S 

iU 

fci    T. 

u 

^2-  o 

*** 

rt    o 

M 

E    = 

£  '3 

o 

£b  9- 

2  3 

3 

.5  13 

.o 

J3 

3  £ 

J 

m  a. 

O 

:s  tc 

a.  m 

o 

CO    \o     ■*• 


o    o    o    0    0     w 


\o      t^  CO      O     0 


■t   m  \o    soo    a»   o 


LESSONS 


IN 


ELEMENTARY    PHYSIOLOGY 


LESSON    I 

A     GENERAL     VIEW     OF      THE     STRUCTURE     AND 
FUNCTIONS    OF    THE    HUMAN    BODY 

1.  The  Work  of  the  Body  as  a  Whole.  —  The  body  of 
a  living  man  performs  a  great  diversity  of  actions,  some  of 
which  are  quite  obvious ;  others  require  more  or  less  careful 
observation;  and  yet  others  can  be  detected  only  by  the 
employment  of  the  most  delicate  appliances  of  science. 

Thus,  some  part  of  the  body  of  a  living  man  is  plainly 
always  in  motion.  Even  in  sleep,  when  the  limbs,  head,  and 
eyelids  may  be  still,  the  incessant  rise  and  fall  of  the  chest 
continue  to  remind  us  that  we  are  viewing  slumber  and  not 
death. 

More  careful  observation,  however,  is  needed  to  detect 
the  motion  of  the  heart ;  or  the  pulsation  of  the  arteries  ;  or 
the  changes  in  the  size  of  the  pupil  of  the  eye  with  varying 
light ;  or  to  ascertain  that  the  air  which  is  breathed  out  of 
the  body  is  hotter  and  damper  than  the  air  which  is  taken 
in  by  breathing. 

And  lastly  :  when  we  try  to  ascertain  what  happens  in  the 
eye  when  that  organ  is  adjusted  to  different  distances;  or 

B  I 


2  ELEMENTARY   PHYSIOLOGY  less, 

what  in  a  nerve  when  it  is  excited  ;  or  of  what  materials  flesh 
and  blood  are  made  ;  or  in  virtue  of  what  mechanism  it  is 
that  a  sudden  pain  makes  one  start  —  we  have  to  call  into 
operation  all  the  methods  of  inductive  and  deductive  logic ; 
all  the  resources  of  physics  and  chemistry ;  and  all  the  deli- 
cacies of  the  art  of  experiment. 

The  sum  of  the  facts  and  generalisations  at  which  we 
arrive  by  these  various  modes  of  inquiry,  be  they  simple  or 
be  they  refined,  concerning  the  actions  of  the  body  and  the 
manner  in  which  those  actions  are  brought  about,  constitutes 
the  science  of  Human  Physiology.  An  elementary  outline 
of  this  science,  and  of  so  much  anatomy  as  is  incidentally 
necessary,  is  the  subject  of  the  following  Lessons  ;  of  which 
we  shall  devote  the  present  to  an  account  of  so  much  of  the 
structure  and  such  of  the  actions  (or,  as  they  are  technically 
called,  "  functions  ")  of  the  body,  as  can  be  ascertained  by 
easy  observation ;  or  might  be  so  ascertained  if  the  bodies 
of  men  were  as  easily  procured,  examined,  and  subjected  to 
experiment,  as  those  of  animals. 

Suppose  a  chamber  with  walls  of  ice,  through  which  a  cur- 
rent of  pure  ice-cold  air  passes  ;  the  walls  of  the  chamber  will 
of  course  remain  unmelted. 

Now,  having  weighed  a  healthy  living  man  with  great  care, 
let  him  walk  up  and  down  the  chamber  for  an  hour.  In  do- 
ing this  he  will  obviously  do  a  considerable  amount  of  work 
and  use  up  a  proportionate  quantity  of  energy  ;  as  much,  at 
least,  as  would  be  required  to  lift  his  weight  as  high  and  as 
often  as  he  has  raised  himself  at  every  step.  But,  in  addi- 
tion, a  certain  quantity  of  the  ice  will  be  melted,  or  converted 
into  water  ;  showing  that  the  man  has  given  off  heat  in  abun- 
dance. Furthermore,  if  the  air  which  enters  the  chamber  be 
made  to  pass  through  lime-water,  it  will  cause  no  cloudy 
white  precipitate  of  carbonate  of  lime,  because  the  quantity 


I  WORK   AND   WASTE  3 

of  carbonic  acid1  in  ordinary  air  is  so  small  as  to  be  inappre- 
ciable in  this  way.  But  if  the  air  which  passes  out  is  made 
to  take  the  same  course,  the  lime-water  will  soon  become 
milky,  from  the  precipitation  of  carbonate  of  lime,  showing 
the  presence  of  carbonic  acid,  which,  like  the  heat,  is  given 
off  by  the  man. 

Again,  even  if  the  air  be  quite  dry  as  it  enters  the  chamber 
(and  the  chamber  be  lined  with  some  material  so  as  to  shut 
out  all  vapour  from  the  melting  ice  walls),  that  which  is 
breathed  out  of  the  man,  and  that  which  is  given  off  from 
his  skin,  will  exhibit  clouds  of  vapour ;  which  vapour,  there- 
fore, is  derived  from  the  body. 

After  the  expiration  of  the  hour  during  which  the  experi- 
ment has  lasted,  let  the  man  be  released  and  weighed  once 
more.     He  will  be  found  to  have  lost  weight. 

Thus  a  living,  active  man  constantly  does  mechanical 
work,  gives  off  heat,  evolves  carbonic  acid  and  water,  and 
undergoes  a  loss  of  substance. 

Plainly,  this  state  of  things  could  not  continue  for  an  un- 
limited period,  or  the  man  would  dwindle  to  nothing.  But 
long  before  the  effects  of  this  gradual  diminution  of  substance 
become  apparent  to  a  bystander,  they  are  felt  by  the  subject 
of  the  experiment  in  the  form  of  the  two  imperious  sensa- 
tions called  hunger  and  thirst.  To  still  these  cravings,  to 
restore  the  weight  of  the  body  to  its  former  amount,  to  ena- 
ble it  to  continue  giving  out  heat,  water,  and  carbonic  acid, 
at  the  same  rate,  for  an  indefinite  period,  it  is  absolutely 
necessary  that  the  body  should  be  supplied  with  each  of 

1  By  "  carbonic  acid  "  we  mean  "  carbonic  acid  gas."  This  should  in 
strictness  be  called  carbon  dioxide  (COo),  carbonic  acid  being  the  com- 
pound of  this  with  water,  H2C03.  But  for  simplicity's  sake,  and  because 
the  expression  "carbonic  acid"  is  in  general  use  and  is  generally  under- 
stood to  stand  for  carbon  dioxide,  we  shall  use  it  throughout  this  book. 


+  ELEMENTARY   PHYSIOLOGY  less. 

three  things,  and  with  three  only.  These  are,  first,  fresh 
air  ;  secondly,  drink  —  consisting  of  water  in  some  shape  or 
other,  however  much  it  may  be  adulterated  ;  thirdly,  food. 
That  compound  known  to  chemists  as  proteid  matter 
(p.  134),  and  which  contains  carbon,  hydrogen,  oxygen, 
and  nitrogen,  must  form  a  part  of  this  food,  if  it  is  to  sustain 
life  indefinitely ;  and  fatty,  starchy,  or  saccharine,  i.e.  car- 
bohydrate matters,  together  with  a  certain  amount  of  salts, 
ought  to  be  contained  in  the  food,  if  it  is  to  sustain  life 
conveniently. 

A  certain  proportion  of  the  matter  taken  in  as  food  either 
cannot  be,  or  at  any  rate  is  not,  used  ;  and  leaves  the  body 
as  excrementitious  matter,  having  simply  passed  through  the 
alimentary  canal  without  undergoing  much  change,  and  with- 
out ever  being  incorporated  into  the  actual  substance  of  the 
body.  But,  under  healthy  conditions,  and  when  only  so 
much  food  as  is  necessary  is  taken,  no  important  proportion 
of  either  proteid  matter,  or  fat,  or  starchy  or  saccharine 
food,  passes  out  of  the  body  as  such.  Almost  all  real  food 
ultimately  leaves  the  body  as  waste  in  the  form  either  of 
•water,  or  of  carbonic  acid,  or  of  a  third  substance  called 
urea,  or  of  certain  saline  compounds  or  salts. 

Chemists  have  determined  that  these  products,  which  are 
thrown  out  of  the  body  and  are  called  excretions,  contain, 
if  taken  together,  far  more  oxygen  than  the  food  and  water 
taken  into  the  body.  Now,  the  only  possible  source  whence 
the  body  can  obtain  oxygen,  except  from  food  and  water, 
is  the  air  which  surrounds  it.1     And  careful  investigation  of 

1  Fresh  country  air  contains  in  every  100  parts  nearly  21  of  oxygen 
and  79  of  nitrogen  gas,  together  with  a  small  fraction  of  a  part  (.04)  of 
carbonic  acid,  and  a  variable  quantity  of  watery  vapour.  The  recently 
discovered  constituent  of  the  atmosphere,  argon,  is  here  reckoned  in  with 
the  nitrogen. 


i  WORK   AND    WASTE  5 

the  air  which  leaves  the  chamber  in  the  imaginary  experi- 
ment described  above  would  show,  not  only  that  it  has 
gained  carbonic  acid  from  the  man,  but  that  it  has  lost 
oxygen  in  equal  or  rather  greater  amount  to  him. 

Thus,  if  a  man  is  neither  gaining  nor  losing  weight,  the 
sum  of  the  weights  of  all  the  substances  above  enumerated 
which  leave  the  body  ought  to  be  exactly  equal  to  the 
weight  of  the  food  and  water  which  enter  it,  together  with 
that  of  the  oxygen  which  it  absorbs  from  the  air.  And  this 
is  proved  to  be  the  case. 

Hence  it  follows  that  a  man  in  health,  and  "  neither  gain- 
ing nor  losing  flesh,"  is  incessantly  oxidating  and  wasting 
away,  and  periodically  making  good  the  loss.  So  that  if, 
in  his  average  condition,  he  could  be  confined  in  the  scale- 
pan  of  a  delicate  spring  balance,  like  that  used  for  weighing 
letters,  the  scale-pan  would  descend  at  every  meal,  and  as- 
cend in  the  intervals,  oscillating  to  equal  distances  on  each 
side  of  the  average  position,  which  would  never  be  main- 
tained for  longer  than  a  few  minutes.  There  is,  therefore, 
no  such  thing  as  a  stationary  condition  of  the  weight  of  the 
body,  and  what  we  call  such  is  simply  a  condition  of  varia- 
tion within  narrow  limits  —  a  condition  in  which  the  gains 
and  losses  of  the  numerous  daily  transactions  of  the  econ- 
omy balance  one  another. 

Suppose  this  diurnally-balanced  physiological  state  to  be 
reached,  it  can  be  maintained  only  so  long  as  the  quantity 
of  the  mechanical  work  done,  and  of  heat,  or  other  force 
evolved,  remains  absolutely  unchanged. 

Let  such  a  physiologically-balanced  man  lift  a  heavy  body 
from  the  ground,  and  the  loss  of  weight  which  he  would 
have  undergone  without  that  exertion  will  be  increased  by 
a  definite  amount,  which  cannot  be  made  good  unless  a  pro- 
portionate amount  of  extra  food  be  supplied  to  him.     Let 


6  ELEMENTARY   PHYSIOLOGY  less, 

the  temperature  of  the  surrounding  air  fall,  and  the  same 
result  will  occur,  if  his  body  remains  as  warm  as  before. 

On  the  other  hand,  diminish  his  exertion  and  lower  his 
production  of  heat,  and  either  he  will  gain  weight,  or  some 
of  his  food  will  remain  unused. 

Thus,  in  a  properly  nourished  man,  a  stream  of  food  is 
constantly  entering  the  body  in  the  shape  of  complex  com- 
pounds containing  comparatively  little  oxygen ;  as  con- 
stantly, the  elements  of  the  food  (whether  before  or  after 
they  have  formed  part  of  the  living  substance)  are  leaving 
the  body,  combined  with  more  oxygen.  And  the  incessant 
breaking  down  and  oxidation  of  the  complex  compounds 
which  enter  the  body  are  definitely  proportioned  to  the 
amount  of  energy  the  body  gives  out,  whether  in  the  shape 
of  heat  or  otherwise  ;  just  in  the  same  way  as  the  amount 
of  work  to  be  got  out  of  a  steam-engine,  and  the  amount  of 
heat  it  and  its  furnace  give  off,  bear  a  strict  proportion  to 
its  consumption  of  fuel. 

From  these  general  considerations  regarding  the  nature 
of  life,  considered  as  physiological  work,  we  may  turn  for 
the  purpose  of  taking  a  like  broad  survey  of  the  apparatus 
which  does  the  work.  We  have  seen  the  general  perfor- 
mance of  the  engine,  we  may  now  look  at  its  build. 

2.  The  General  Build  of  the  Body.  —  The  human  body 
is  obviously  separable  into  head,  trunk,  and  limbs.  In  the 
head,  the  brain-case  or  skull  is  distinguishable  from  the 
face.  The  trunk  is  naturally  divided  into  the  chest  or 
thorax,  and  the  belly  or  abdomen.  Of  the  limbs  there  are 
two  pairs  - —  the  upper,  or  arms,  and  the  lower,  or  legs  ;  and 
legs  and  arms  again  are  subdivided  by  their  joints  into  parts 
which  obviously  exhibit  a  rough  correspondence  —  thigh 
and  upper  arm,  leg  and  fore -arm,  ankle  and  wrist,  toes  and 
fingers,  plainly  answering  to  one  another.    And  the  two  last, 


1  THE   BUILD   OF  THE   BODY  7 

in  fact,  are  so  similar  that  they  receive  the  same  name  of 
digits  ;  while  the  several  joints  of  the  fingers  and  toes  have 
the  common  denomination  of  phalanges. 

The  whole  body  thus  composed  (without  the  viscera  or 
organs  which  fill  the  cavities  of  the  trunk)  is  seen  to  be 
bilaterally  symmetrical ;  that  is  to  say,  if  it  were  split  length- 
wise by  a  great  knife,  which  should  be  made  to  pass  along 
the  middle  line  of  both  the  dorsal  and  ventral  (or  back  and 
front)  aspects,  the  two  halves  would  almost  exactly  resemble 
one  another. 

One-half  of  the  body,  divided  in  the  manner  described 
(Fig.  1,  A),  would  exhibit,  in  the  trunk,  the  cut  faces  of 
thirty-three  bones,  joined  together  by  a  very  strong  and 
tough  substance  into  a  long  column,  which  lies  much  nearer 
the  dorsal  (or  back)  than  the  ventral  (or  front)  aspect  of 
the  body.  The  bones  thus  cut  through  are  called  the  bodies 
of  the  vertebrae.  They  separate  a  long,  narrow  canal,  called 
the  spinal  canal,  which  is  placed  upon  their  dorsal  side, 
from  the  spacious  chamber  of  the  chest  and  abdomen,  which 
lies  upon  their  ventral  side.  There  is  no  direct  communi- 
cation between  the  dorsal  canal  and  the  ventral  cavity. 

The  spinal  canal  contains  a  long  white  cord  —  the  spinal 
cord  —  which  is  an  important  part  of  the  nervous  system. 
The  ventral  chamber  is  divided  into  the  two  subordinate 
cavities  of  the  thorax  and  abdomen  by  a  remarkable,  partly 
fleshy  and  partly  membranous,  partition,  the  diaphragm 
(Fig.  1,  D),  which  is  concave  towards  the  abdomen,  and 
convex  towards  the  thorax.  The  alimentary  canal  (Fig.  1, 
A/.)  traverses  these  cavities  from  one  end  to  the  other, 
piercing  the  diaphragm.  So  does  a  long  double  series  ot 
distinct  masses  of  nervous  substance,  which  are  called 
ganglia :  these  are  connected  together  by  nervous  cords, 
and  constitute  the  so-called  sympathetic  system  (Fig.  1,  Sy). 


8  ELEMENTARY   PHYSIOLOGY  less. 

The  abdomen  contains,  in  addition  to  these  parts,  the 
two  kidneys,  one  placed  against  each  side  of  the  vertebral 
column  and  connected  each  by  a  tube,  the  ureter,  to  a 
muscular  bag,  the  bladder,  lying  at  the  bottom  of  the  abdo- 
men;  the  liver,  the  pancreas  or  "sweetbread,"  and  the 
spleen.  The  thorax  incloses,  besides  its  segment  of  the  ali- 
mentary canal  and  of  the  sympathetic  system,  the  heart 
and  the  two  lungs.  The  latter  are  placed  one  on  each  side 
of  the  heart,  which  lies  nearly  in  the  middle  of  the  thorax. 

Where  the  body  is  succeeded  by  the  head,  the  upper- 
most of  the  thirty-three  vertebral  bodies  is  followed  by  a 
continuous  mass  of  bone,  which  extends  through  the  whole 
length  of  the  head,  and,  like  the  spinal  column,  separates  a 
dorsal  chamber  from  a  ventral  one.  The  dorsal  chamber, 
or  cavity  of  the  skull,  opens  into  the  spinal  canal.  It  con- 
tains a  mass  of  nervous  matter  called  the  brain,  which  is  con- 
tinuous with  the  spinal  cord,  the  brain  and  the  spinal  cord 
together  constituting  what  is  termed  the  cerebro-spinal  sys- 
tem (Fig.  i,  C.S.,  C.S.).  The  ventral  chamber,  or  cavity 
of  the  face,  is  almost  entirely  occupied  by  the  mouth  and 
pharynx,  into  which  last  the  upper  end  of  the  alimentary 
canal  (called  gullet  or  oesophagus)  opens. 

Thus,  the  study  of  a  longitudinal  section  shows  us  that 
the  human  body  is  a  double  tube,  the  two  tubes  being  com- 
pletely separated  by  the  spinal  column  and  the  bony  axis  of 
the  skull,  which  form  the  floor  of  the  one  tube  and  the  roof 
of  the  other.  The  dorsal  tube  contains  the  cerebro-spinal 
axis ;  the  ventral  tube  contains  the.  alimentary  canal,  the 
sympathetic  nervous  system,  the  heart,  and  the  lungs,  besides 
other  organs. 

Transverse  sections,  taken  perpendicularly  to  the  axis  of 
the  vertebral  column  or  to  that  of  the  skull,  show  still  more 
clearly  that  this  is  the  fundamental  structure  of  the  human 


THE   BUILD   OF  THE   BODY 


Fig.  i. 

A.  A  diagrammatic  section  of  the  human  body,  taken  vertically  through  the 
median  plane.  C.S. ,  the  cerebro-spinal  nervous  system;  JV,  the  cavity  of  the  nose; 
M,  that  of  the  mouth;  Al.,AL,  the  alimentary  canal  represented  as  a  simple  tube; 
H,  the  heart;  D,  the  diaphragm;   Sy,  the  sympathetic  ganglia. 

B.  A  transverse  vertical  section  of  the  head  taken  along  the  line  a  b;  letters  as 
before. 

C.  A  transverse  section  taken  along  the  line  c  d;   letters  as  before. 


io  ELEMENTARY   PHYSIOLOGY  less. 

body,  and  that  the  great  apparent  difference  between  the 
head  and  the  trunk  is  due  to  the  different  size  of  the  dorsal 
cavity  relatively  to  the  ventral.  In  the  head  the  former 
cavity  is  very  large  in  proportion  to  the  size  of  the  latter 
(Fig.  i,  B)  ;  in  the  thorax  or  abdomen  it  is  very  small 
(Fig.  i,  C). 

The  limbs  contain  no  such  chambers  as  are  found  in  the 
body  and  the  head ;  but  with  the  exception  of  certain 
branching  tubes  filled  with  fluid,  which  are  called  blood- 
vessels and  lymphatics,  are  solid  or  semi-solid,  throughout. 

3.  The  Tissues.  —  Such  being  the  general  character  and 
arrangement  of  the  parts  of  the  human  body,  it  will  next  be 
well  to  consider  into  what  constituents  it  may  be  separated 
by  the  aid  of  no  better  means  of  discrimination  than  the 
eye  and  the  anatomist's  knife. 

With  no  more  elaborate  aids  than  these,  it  becomes  easy 
to  separate  that  tough  membrane  which  invests  the  whole 
body,  and  is  called  the  skin,  or  integument,  from  the  parts 
which  lie  beneath  it.  Furthermore,  it  is  readily  enough 
ascertained  that  this  integument  consists  of  two  portions  : 
a  superficial  layer,  which  is  constantly  being  shed  in  the 
form  of  powder  or  scales,  composed  of  minute  particles  of 
horny  matter,  and  is  called  the  epidermis ;  and  the  deeper 
part,  the  dermis,  which  is  dense  and  fibrous  (p.  215). 
The  epidermis,  if  wounded,  neither  gives  rise  to  pain  nor 
bleeds.  The  dermis,  under  like  circumstances,  is  very  ten- 
der, and  bleeds  freely.  A  practical  distinction  is  drawn 
between  the  two  in  shaving,  in  the  course  of  which  opera- 
tion the  razor  ought  to  cut  only  epidermal  structures ;  for  if 
it  go  a  shade  deeper,  it  gives  rise  to  pain  and  bleeding. 

The  skin  can  be  readily  enough  removed  from  all  parts 
of  the  exterior,  but  at  the  margins  of  the  apertures  of  the 
body  it  seems  to  stop,  and  to  be  replaced  by  a  layer  which 


I  THE  TISSUES  " 

is  much  redder,  more  sensitive,  bleeds  more  readily,  and 
which  keeps  itself  continually  moist  by  giving  out  a  more  or 
less  tenacious  fluid,  called  mucus.  Hence,  at  these  aper- 
tures, the  skin  is  said  to  stop,  and  to  be  replaced  by  mucous 
membrane,  which  lines  all  those  interior  cavities,  such  as  the 
alimentary  canal,  into  which  the  apertures  open.  But,  in 
truth,  the  skin  does  not  really  come  to  an  end  at  these  points, 
but  is  directly  continued  into  the  mucous  membrane,  which 
last  is  simply  an  integument  of  greater  delicacy,  but  consist- 
ing fundamentally  of  the  same  two  layers  —  a  deep,  fibrous 
layer,  called  also  dermis,  and  containing  blood-vessels,  and  a 
superficial,  bloodless  one,  now  called  the  epithelium.  Thus 
every  part  of  the  body  might  be  said  to  be  contained  between 
the  walls  of  a  double  bag,  formed  by  the  epidermis,  which 
invests  the  outside  of  the  body,  and  the  epithelium,  its  con- 
tinuation, which  lines  the  alimentary  canal. 

The  dermis  of  the  skin,  and  that  of  the  mucous  mem- 
branes, are  chiefly  made  up  of  a  filamentous  substance, 
which  yields  abundant  gelatine  on  being  boiled,  and  is  the 
matter  which  tans  when  hide  is  made  into  leather.  This  is 
called  connective  tissue,1  because  it  is  the  great  connecting 
medium  by  which  the  different  parts  of  the  body  are  held 
together.  Thus  it  passes  from  the  dermis  between  all  the 
other  organs,  ensheathing  the  muscles,  coating  the  bones  and 
cartilages,  and  eventually  reaching  and  entering  into  the 
mucous  membranes.  And  so  completely  and  thoroughly 
does  the  connective  tissue  permeate  almost  all  parts  of  the 
body,  that,  if  every  other  tissue  could  be  dissected  away,  a 
complete  model  of  all  the  organs  would  be  left  composed  of 
this  tissue.  Connective  tissue  varies  very  much  in  character  ; 
in  some  places  being  very  soft  and  tender,  at  others  —  as  in 

1  Every  such  constituent  of  the  body,  as  epidermis,  cartilage,  or  muscle, 
is  called  a  "  tissue."     (See  Lesson  II.) 


12  ELEMENTARY   PHYSIOLOGY  less. 

the  tendons  and  ligaments,  which  are  almost  wholly  com- 
posed of  it  —  attaining  great  strength  and  density. 

Among  the  most  important  of  the  tissues  imbedded  in 
and  ensheathed  by  the  connective  tissue,  are  some  the  pres- 
ence and  action  of  which  can  be  readily  determined  during 
life. 

If  the  upper  arm  of  a  man  whose  arm  is  stretched  out 
be  tightly  grasped  by  another  person,  the  latter,  as  the 
former  bends  up  his  fore-arm,  will  feel  a  great  soft  mass, 
which  lies  at  the  fore  part  of  the  upper  arm,  swell,  harden, 
and  become  prominent.  As  the  arm  is  extended  again,  the 
swelling  and  hardness  vanish. 

On  removing  the  skin,  the  body  which  thus  changes  its 
configuration  is  found  to  be  a  mass  of  red  flesh,  sheathed  in 
connective  tissue.  The  sheath  is  continued  at  each  end  into 
a  tendon,  by  which  the  muscle  is  attached,  on  the  one  hand, 
to  the  shoulder-bone,  and,  on  the  other,  to  one  of  the  bones 
of  the  fore-arm.  This  mass  of  flesh  is  the  muscle  called 
biceps,  and  it  has  the  peculiar  property  of  changing  its  dimen- 
sions —  shortening  and  becoming  thick  in  proportion  to  its 
decrease  in  length  —  when  influenced  by  the  will  as  well  as 
by  some  other  causes,  called  stimuli,  and  of  returning  to  its 
original  form  when  let  alone.  This  temporary  change  in  the 
dimensions  of  a  muscle,  this  shortening  and  thickening,  is 
spoken  of  as  its  contraction.  It  is  by  reason  of  this  property 
that  muscular  tissue  becomes  the  great  motor  agent  of  the 
body ;  the  muscles  being  so  disposed  between  the  systems 
of  levers  which  support  the  body,  that  their  contraction 
necessitates  the  motion  of  one  lever  upon  another. 

4.  The  Skeleton.  —  These  levers  form  part  of  the  system 
of  hard  tissues  which  constitute  the  skeleton.  The  less  hard 
of  these  are  the  cartilages,  composed  of  a  dense,  firm  sub- 
stance, ordinarily  known  as  "gristle."     The  harder  are  the 


THE    SKELETON 


*3 


Fig.  2.  —  The  Vertebral  Column. 

A,  side  view,  left  side;  B,  back  view;  C  1-7,  cervical  vertebra:;  D  1-12,  dors?! 
(thoracic)  vertebrae;  L  1-5,  lumbar  vertebras;  S,  sacrum;  C,  coccyx;  sp,  spinous 
processes;  tr,  transverse  processes. 


14 


ELEM ENTARY   PHYSIOLOGY 


bones,  which  are  masses  of  tissue,  hardened  by  being  impreg- 
nated with  phosphate  and  carbonate  of  lime.  They  are  ani- 
mal tissues  which  have  become,  in  a  manner,  naturally 
petrified  ;  and  when  the  salts  of  lime  are  extracted,  as  they 
may  be,  by  the  action  of  acids,  a  model  of  the  bone  in 
soft  and  flexible  animal  matter  remains. 


Fig.  3.  —  Side  View  of  the  Skull. 

/,  frontal  bone;  /,  parietal;  o,  occipital;  a,  wing  of  sphenoid;  s,  flat  part  of  tem- 
poral; c,  vt,  st,  other  parts  of  temporal;  an,  opening  of  ear  or  external  auditory 
canal;  z,  process  of  temporal  passing  to./',  the  cheek-bone;  mx,  the  upper  jaw-bone; 
■n,  nasal  bone;  /,  lacrymal;  pi,  part  of  sphenoid.  The  lower  jaw-bone  is  drawn 
downwards;  cy,  its  process  which  articulates  with  the  temporal;  cr,  its  process  to 
which  muscles  of  mastication  are  attached;  th,  ty,  hyoid  bone,  the  dotted  line  indi- 
cating its  attachment  by  a  ligament  to  the  temporal. 

More  than  200  separate  bones  are  ordinarily  reckoned  in 
the  human  body,  though  the  actual  number  of  distinct  bones 
varies  at  different  periods  of  life,  many  bones  which  are 
separate   in   youth  becoming  united   together  in   old  age 


THE   SKELETON 


15 


Thus  there  are  originally,  as  we  have  seen,  thirty-three 
separate  bodies  of  vertebrae  in  the  spinal  column  (Fig.  2), 
and  the  upper  twenty-four  of  these  commonly  remain  dis- 
tinct throughout  life.  But  the  twenty-fifth,  twenty-sixth, 
twenty-seventh,  twenty-eighth,  and  twenty-ninth  early  unite 
into  one  great  bone,  called  the  sacrum;  and  the  four  remain- 
ing vertebrae  often  run  into  one  bony  mass  called  the 
coccyx. 


SLIT 


disc 


Fig. 


-  The  Pelvis 


Sac,  sacrum;  Cocc,  coccyx;  il,  is,  pu,  ilium,  ischium,  pubis,  three  parts  of  ths 
innominate  or  hip-bone;  acet,  acetabulum  or  cup  for  head  of"  femur;  j L.  F,  5th  lum- 
bar vertebra;  (fee,  disc  of  cartilage  between  vertebrae;   R,  right;   L,  left. 


In  early  adult  life,  the  skull  contains  twenty-two  naturally 
separate  bones,  but  in  youth  the  number  is  much  greater, 
and  in  old  age  far  less. 

Twenty-four  ribs  bound  the  chest  laterally,  twelve  on  each 
side,  and  most  of  them  are  connected  by  cartilages  with  the 
breast-bone  or  sternum   (Fig.  50,  p.    161).      In  the  girdle 


n6 


ELEMENTARY   PHYSIOLOGY 
cl 


hum 


.rad. 


Fig.  5.  —  The  Bones  ok  -iiiic  Limbs.     Front  View.     Left  Limbs. 

A,  the  innominate  and  bones  of  the  leg;  inn,  innominate  or  hip-bone ;fem,  femur; 
pat,  patella  or  knee-cap;  tit>,  tibia;y?/<,  fibula;  tar,  (seven)  tarsal  bones;  metat,  (five) 
metatarsal  bones;  phi,  (fourteen)  phalanges.  B,  the  scapula,  clavicle,  and  bones  ol 
the  arm;  cl,  clavicle  or  collar-bone;  scap,  scapula  or  shoulder-bone;  hum,  humerus; 
rdd, radius;  iilu,  ulna;  car,  (eight)  carpal  bones;  metar,  (five)  metacarpal  bones; 
phi.  (fourteen)  phalanges. 


i  THE   ERECT   POSITION  17 

which  supports  the  shoulder,  two  bones  are  always  distin- 
guishable as  the  scapula,  or  shoulder-blade,  and  the  clavicle, 
or  collar-bone  (Fig.  5,  B).  The  pelvis  (Fig.  4),  to  which 
the  legs  are  attached,  consists  of  two  separate  bones  called 
the  ossa  innominata,  or  hip-bones,  in  the  adult ;  but  each  os 
innominatum  is  separable  into  three  (called  pubis,  ischium, 
and  ilium)  in  the  young. 

There  are  thirty  bones  in  each  of  the  arms,  and  the  same 
number  in  each  of  the  legs,  counting  the  patella,  or  knee- 
pan  (Fig.  5). 

All  these  bones  are  fastened  together  by  ligaments,  or  by 
cartilages  ;  and  where  they  play  freely  over  one  another,  a 
coat  of  cartilage  furnishes  the  surfaces  which  come  into  con- 
tact. The  cartilages  which  thus  form  part  of  a  joint  are 
called  articular  cartilages,  and  their  free  surfaces,  by  which 
they  rub  against  each  other,  are  lined  by  a  delicate  syno- 
vial membrane,  which  secretes  a  lubricating  fluid  —  the 
synovia. 

5.  The  Erect  Position.  —  Though  the  bones  of  the  skele- 
ton are  all  strongly  enough  connected  together  by  ligaments 
and  cartilages,  the  joints  play  so  freely,  and  the  centre  of  grav- 
ity of  the  body,  when  erect,  is  so  high  up,  that  it  is  impossi- 
ble to  make  a  skeleton  or  a  dead  body  support  itself  in  the 
upright  position.  That  position,  easy  as  it  seems,  is  the 
result  of  the  contraction  of  a  multitude  of  muscles  which 
oppose  and  balance  one  another.  Thus,  the  foot  affording 
the  surface  of  support,  the  muscles  of  the  calf  (Fig.  6,  I) 
must  contract,  or  the  legs  and  body  would  fall  forward.  But 
this  action  tends  to  bend  the  leg ;  and  to  neutralise  this  and 
keep  the  leg  straight,  the  great  muscles  in  front  of  the  thigh 
(Fig.  6,  2)  must  come  into  play.  But  these,  by  the  same 
action,  tend  to  bend  the  body  forward  on  the  legs  ;  and  if 
the  body  is  to  be  kept  straight,  they  must  be  neutralised  by 

G 


i8  ELEMENTARY   PHYSIOLOGY  less. 

the  action  of  the  muscles  of  the  buttocks  and  of  the  back 
(Fig.  6,  III). 

The  erect  position,  then,  which  we  assume  so  easily  and 
without  thinking  about  it,  is  the  result  of  the  combined  and 


i*'io.  6.  —  A  Diagram   illustrating  the  Attachments   of  some  of  the  most 
important  Muscles  which  keep  the  Body  in  the  Erect  Posture. 

I.  The  muscles  of  the  calf.  II.  Those  of  the  back  of  the  thigh.  III.  Those  of 
the  spine.     These  tend  to  keep  the  body  from  falling  forward. 

r.  The  muscles  of  the  front  of  the  leg.  2.  Those  of  the  front  of  the  thigh. 
3.  Those  of  the  front  of  the  abdomen.  4,  5.  Those  of  the  front  of  the  neck.  These 
tend  to  keep  the  body  from  falling  backward.  The  arrows  indicate  the  direction  o( 
action  of  the  muscles,  the  foot  being  fixed. 


I  THE   ERECT   POSITION  19 

accurately  proportioned  action  of  a  vast  number  of  muscles. 
What  is  it  that  makes  them  work  together  in  this  way  ? 

Let  any  person  in  the  erect  position  receive  a  violent 
blow  on  the  head,  and  you  know  what  occurs.  On  the 
instant  he  drops  prostrate,  in  a  heap,  with  his  limbs  relaxed 
and  powerless.  What  has  happened  to  him?  The  blow 
may  have  been  so  inflicted  as  not  to  touch  a  single  muscle 
of  the  body;  it  may  not  cause  the  loss  of  a  drop  of  blood  ; 
and,  indeed,  if  the  "  concussion,"  as  it  is  called,  has  not  been 
too  severe,  the  sufferer,  after  a  few  moments  of  unconscious- 
ness, will  come  to  himself,  and  be  as  well  as  ever  again. 
Clearly,  therefore,  no  permanent  injury  has  been  done  to 
any  part  of  the  body,  least  of  all  to  the  muscles,  but  an  influ- 
ence has  been  exerted  upon  a  something  which  governs  the 
muscles.  And  a  similar  influence  may  be  the  effect  of  very 
subtle  causes.  A  strong  mental  emotion,  and  even  a  very 
bad  smell,  will,  in  some  people,  produce  the  same  effect  as 
a  blow. 

These  observations  might  lead  to  the  conclusion  that  it  is 
the  mind  which  directly  governs  the  muscles,  but  a  little 
further  inquiry  will  show  that  such  is  not  the  case.  For 
people  have  been  so  stabbed,  or  shot  in  the  back,  as  to  cut 
the  spinal  cord,  without  any  considerable  injury  to  other 
parts :  and  then  they  have  lost  the  power  of  standing 
upright  as  much  as  before,  though  their  minds  may  have 
remained  perfectly  clear.  And  not  only  have  they  lost  the 
power  of  standing  upright  under  these  circumstances,  but 
they  no  longer  retain  any  power  of  either  feeling  what  is 
going  on  in  their  legs,  or,  by  an  act  of  their  own  will,  causing 
motion  in  them. 

And  yet,  though  the  mind  is  thus  cut  off  from  the  lower 
limbs,  a  controlling  and  governing  power  over  them  still  re- 
mains in  the  body.     For  if  the  soles  of  the  disabled  feet  be 


20  ELEMENTARY   PHYSIOLOGY  less. 

tickled,  though  the  mind  does  not  feel  the  tickling,  the  legs 
will  be  jerked  up,  just  as  would  be  the  case  in  an  uninjured 
person.  Again,  if  a  series  of  galvanic  shocks  be  sent  into 
the  spinal  cord,  the  legs  will  perform  movements  even  more 
powerful  than  those  which  the  will  could  produce  in  an  unin- 
jured person.  And,  finally,  if  the  injury  is  of  such  a  nature 
as  not  simply  to  divide  or  injure  the  spinal  cord  in  one 
place  only,  but  to  crush  or  profoundly  disorganise  it,  all 
these  phenomena  cease  ;  tickling  the  soles,  or  sending  gal- 
vanic shocks  along  the  spine,  will  produce  no  effect  upon 
the  legs. 

By  examinations  of  this  kind  carried  still  further,  we  arrive 
at  the  remarkable  result  that,  while  the  brain  is  the  seat  of 
all  sensation  and  mental  action,  and  the  primary  source  of 
all  voluntary  muscular  contractions,  the  spinal  cord  is  by 
itself  capable  of  receiving  an  impression  from  the  exterior, 
and  converting  it,  not  only  into  a  simple  muscular  contrac- 
tion, but  into  a  combination  of  such  actions. 

Thus,  in  general  terms,  we  may  say  of  the  cerebro-spinal 
nervous  centres,  that  they  have  the  power,  when  they  receive 
certain  impressions  from  without,  of  giving  rise  to  simple  or 
combined  muscular  contractions. 

6.  Sensory  Organs.  —  But  you  will  further  note  that  these 
impressions  from  without  are  of  very  different  characters. 
Any  part  of  the  surface  of  the  body  may  be  so  affected  as  to 
give  rise  to  the  sensations  of  contact,  or  of  heat  or  cold ; 
and  any  or  every  substance  is  able,  under  certain  circum- 
stances, to  produce  these  sensations.  But  only  very  few 
and  comparatively  small  portions  of  the  bodily  framework 
are  competent  to  be  affected  in  such  a  manner  as  to  cause 
the  sensations  of  taste  or  of  smell,  of  sight  or  of  hearing  : 
and  only  a  few  substances,  or  particular  kinds  of  vibrations, 
are  able  so  to  affect  those  regions.     These  very  limited  parts 


I  THE  ORGANS  ?i 

of  the  body,  which  put  us  in  relation  with  particular  kinds 
of  substances,  or  forms  of  force,  are  what  are  termed  sen- 
sory organs.  There  are  two  such  organs  for  sight,  two  for 
hearing,  two  for  smell,  and  one,  or  more  strictly  speaking 
two,  for  taste. 

7.  The  Renewal  of  the  Tissues.  —  And  now  that  we  have 
taken  this  brief  view  of  the  structure  of  the  body,  of  the 
organs  which  support  it,  of  the  organs  which  move  it,  and 
of  the  organs  which  put  it  in  relation  with  the  surrounding 
world,  or,  in  other  words,  enable  it  to  move  in  harmony 
with  influences  from  without,  we  must  consider  the  means 
by  which  all  this  wonderful  apparatus  is  kept  in  working 
order. 

All  work,  as  we  have  seen,  implies  waste.  The  work  of 
the  nervous  system  and  that  of  the  muscles,  therefore,  im- 
plies consumption  either  of  their  own  substance  or  of  some- 
thing else.  And  as  the  organism  can  make  nothing,  it  must 
possess  the  means  of  obtaining  from  without  that  which  it 
wants,  and  of  throwing  off  from  itself  that  which  it  wastes; 
and  we  have  seen  that,  in  the  gross,  it  does  these  things.  The 
body  feeds,  and  it  excretes.  But  we  must  now  pass  from  the 
broad  fact  to  the  mechanism  by  which  the  fact  is  brought 
about.  The'organs  which  convert  food  into  nutriment  are 
the  organs  of  alimentation ;  those  which  distribute  nutri- 
ment all  over  the  body  are  organs  of  circulation ;  those 
which  get  rid  of  the  waste  products  are  organs  of  excretion. 

8.  Alimentary  Organs,  —  The  organs  of  alimentation  are 
the  mouth,  pharynx,  gullet,  stomach,  and  intestines,  with 
their  appendages,  the  pancreas  and  the  liver.  What  they 
do  is,  first,  to  receive  and  grind  the  food.  They  then  act 
upon  it  with  chemical  agents,  of  which  they  possess  a  store 
which  is  renewed  as  fast  as  it  is  used ;  and  in  this  way 
convert  the  food  by  processes  of  digestion  into  a  fluid  con- 


22  ELEMENTARY   PHYSIOLOGY  less. 

taining  nutritious  matters  in  solution  or  suspension,  and 
innutritious  dregs  or  faeces. 

9.  Circulatory  Organs.  —  A  system  of  minute  tubes, 
with  very  thin  walls,  termed  capillaries,  is  distributed 
through  the  whole  organism  except  the  epidermis  and  its 
products,  the  epithelium,  the  cartilages,  and  the  substance 
of  the  teeth.  On  all  sides,  these  tubes  pass  into  others, 
which  are  called  arteries  and  veins  ;  while  these,  becoming 
larger  and  larger,  at  length  open  into  the  heart,  an  organ 
which,  as  we  have  seen,  is  placed  in  the  thorax.  During 
life,  these  tubes  and  the  chambers  of  the  heart,  with  which 
they  are  connected,  are  all  full  of  liquid,  which  is,  for  the 
most  part,  that  red  fluid  with  which  we  are  all  familiar  as 
blood. 

The  walls  of  the  heart  are  muscular,  and  contract  rhyth- 
mically, or  at  regular  intervals.  By  means  of  these  contrac- 
tions the  blood  which  its  cavities  contain  is  driven  in  jets 
out  of  these  cavities,  into  the  arteries,  and  thence  into  the 
capillaries,  whence  it  returns  by  the  veins  back  into  the 
heart. 

This  is  the  circulation  of  the  blood. 

Now  the  fluid  containing  the  dissolved  or  suspended  nu- 
tritive matters  which  are  the  result  of  the  process  of  diges- 
tion, traverses  the  very  thin  layer  of  soft  and  permeable 
tissue  which  separates  the  cavity  of  the  alimentary  canal 
from  the  cavities  of  the  innumerable  capillary  vessels  which 
lie  in  the  walls  of  that  canal,  and  so  enters  the  blood,  with 
which  those  capillaries  are  filled.  Whirled  away  by  the  tor- 
rent of  the  circulation,  the  blood,  thus  charged  with  nutri- 
tive matter,  enters  the  heart,  and  is  thence  propelled  into 
the  organs  of  the  body.  To  these  organs  it  supplies  the 
nutriment  with  which  it  is  charged  ;  from  them  it  takes  their 
waste  products,  and,  finally,   returns   by  the  veins  to  the 


I  THE  ORGANS  23 

heart,  loaded  with  useless  and  injurious  excretions,  which 
sooner  or  later  take  the  form  of  water,  carbonic  acid,  and 
urea. 

10.  Excretory  Organs.  —  These  excretionary  matters  are 
separated  from  the  blood  by  the  excretory  organs,  of  which 
there  are  three  —  the  skin,  the  lungs,  and  the  kidneys. 

Different  as  these  organs  may  be  in  appearance,  they  are 
constructed  upon  one  and  the  same  principle.  Each,  in 
ultimate  analysis,  consists  of  a  very  thin  sheet  of  tissue, 
like  so  much  delicate  blotting-paper,  the  one  face  of  which 
is  free,  or  lines  a  cavity  in  communication  with  the  exterior 
of  the  body,  while  the  other  is  in  contact  with  the  blood 
which  has  to  be  purified. 

The  excreted  matters  are,  as  it  were  (though,  as  we  shall 
see,  in  a  peculiar  way),  strained  from  the  blood,  through  this 
delicate  layer  of  tissue,  and  on  to  its  free  surface,  whence 
they  make  their  escape. 

Each  of  these  organs  is  especially  concerned  in  the  elimi- 
nation of  one  of  the  chief  waste  products  —  water,  carbonic 
acid,  and  urea  —  though  it  may  at  the  same  time  be  a  means 
of  escape  for  the  others.  Thus,  the  lungs  are  especially 
busied  in  getting  rid  of  carbonic  acid,  but  at  the  same  time 
they  give  off  a  good  deal  of  water.  The  duty  of  the  kidneys 
is  to  excrete  urea  (together  with  other  substances,  chiefly 
salts),  but  at  the  same  time  they  pass  away  a  large  quan- 
tity of  water  and  a  trifling  amount  of  carbonic  acid ; 
while  the  skin  gives  off  much  water,  some  carbonic  acid, 
and  a  certain  quantity  of  saline  matter,  among  which  a 
trace  of  urea  may  be,  sometimes,  though  very  doubtfully, 
present. 

11.  Respiratory  Organs.  —  Finally,  the  lungs  play  a 
double  part,  being  not  merely  eliminators  of  waste,  or 
excretionary  products,  but  importers  into  the  economy  of 


24  ELEMENTARY    PHYSIOLOGY  less 

a  substance  which  is  not  exactly  either  food  or  drink,  but 
something  as  important  as  either,  —  to  wit,  oxygen. 

As  the  carbonic  acid  (and  water)  is  passing  from  the 
blood  through  the  lungs  into  the  external  air,  oxygen  is  pass- 
ing from  the  air  through  the  lungs  into  the  blood,  and  is 
carried,  as  we  shall  see,  by  the  blood  lo  all  parts  of  the  body. 
We  have  seen  (p.  4)  that  the  waste  which  leaves  the  body 
contains  more  oxygen  than  the  food  which  enters  the 
body.  Indeed  oxidation,  the  oxygen  being  supplied  by  the 
blood,  is  going  on  all  over  the  body.  All  parts  of  the  body  are 
thus  continually  being  oxidised,  or,  in  other  words,  are  con- 
tinually burning,  some  more  rapidly  and  fiercely  than  others. 
And  this  burning,  though  it  is  carried  on  in  a  peculiar  man- 
ner, so  as  never  to  give  rise  to  a  flame,  yet  nevertheless  pro- 
duces an  amount  of  heat  which  is  as  efficient  as  a  fire  to 
raise  the  blood  to  a  temperature  of  about  370  C.  (98. 6°  F.)  ; 
and  this  hot  fluid,  incessantly  renewed  in  all  parts  of  the 
body  by  the  torrent  of  the  circulation,  warms  the  body,  as  a 
house  is  warmed  by  hot-water  apparatus.  Nor  is  it  alone 
the  heat  of  the  body  which  is  provided  by  this  oxidation ; 
the  energy  which  appears  in  the  muscular  work  done  by  the 
body  has  the  same  source.  Just  as  the  burning  of  the  coal 
in  a  steam-engine  supplies  the  motive  power  which  drives 
the  wheels,  so,  though  in  a  peculiar  way,  the  oxidation  of  the 
muscles  (and  thus  ultimately  of  the  food)  supplies  the  motive 
power  of  those  muscular  contractions  which  carry  out  the 
movements  of  the  body.  The  food,  like  coal  combustible  or 
capable  of  oxidation,  is  built  up  into  the  living  body,  which, 
in  like  manner  combustible,  is  continually  being  oxidised  by 
the  oxygen  from  the  blood,  thus  doing  work  and  giving  out 
heat.  Some  of  the  food  perhaps  may  be  oxidised  without 
ever  actually  forming  part  of  the  body  or  after  it  has  already 
become  waste  matter,  but  this  does  not  concern  us  now. 


I  LIFE   AND    DEATH  25 

12.  Coordinating  Action  of  the  Nervous  System.  — 
These  alimentary,  circulatory  or  distributive,  excretory,  and 
respiratory  (oxidational)  processes  would  however  be  worse 
than  useless  if  they  were  not  kept  in  strict  proportion  one  to 
another.  If  the  state  of  physiological  balance  is  to  be  main- 
tained, not  only  must  the  quantity  of  food  taken  be  at  least 
equivalent  to  the  quantity  of  matter  excreted  ;  but  that  food 
must  be  distributed  with  due  rapidity  to  the  seat  of  each 
local  waste.  The  circulatory  system  is  the  commissariat  of 
the  physiological  army. 

Again,  if  the  body  is  to  be  maintained  at  a  tolerably  even 
temperature,  while  that  of  the  air  is  constantly  varying,  the 
condition  of  the  hot-water  apparatus  must  be  most  carefully 
regulated. 

In  other  words,  a  coordinating  organ  must  be  added 
to  the  organs  already  mentioned,  and  this  is  found  in  the 
nervous  system,  which  not  only  possesses  the  function 
already  described  of  enabling  us  to  move  our  bodies  and  to 
know  what  is  going  on  in  the  external  world  ;  but  makes  us 
aware  of  the  need  of  food,  enables  us  to  discriminate  nutri- 
tious from  innutritious  matters,  and  to  exert  the  muscular 
actions  needful  for  seizing,  killing,  and  cooking  ;  guides  the 
hand  to  the  mouth,  governs  all  the  movements  of  the  jaws 
and  of  the  alimentary  canal,  and  determines  the  due  supply 
of  the  juices  necessary  for  digestion.  By  it,  the  working  of 
the  heart  is  properly  adjusted  and  the  calibers  of  the  dis- 
tributing pipes  are  regulated,  so  as  indirectly  to  govern  the 
excretory  and  oxidational  processes,  which  are  also  addi- 
tionally and  more  directly  affected  by  other  actions  of  the 
nervous  system. 

13.  Life  and  Death.  —  The  various  functions  which  have 
been  thus  briefly  indicated  constitute  the  greater  part  of  what 
are  called  the  vital  actions  of  the  human  body,  and  so  long 


26  ELEMENTARY   PHYSIOLOGY  less. 

as  they  are  performed,  the  body  is  said  to  possess  life.  The 
cessation  of  the  performance  of  these  functions  is  what  is 
ordinarily  called  death. 

But  there  are  really  several  kinds  of  death,  which  may,  in 
the  first  place,  be  distinguished  from  one  another  under  the 
two  heads  of  local  and  of  general  death. 

(i)  Local  death  is  going  on  at  every  moment,  and  in  most, 
if  not  in  all,  parts  of  the  living  body.  Individual  cells  of  the 
epidermis  and  of  the  epithelium  are  incessantly  dying  and 
being  cast  off,  to  be  replaced  by  others  which  are,  as  con- 
stantly, coming  into  separate  existence.  The  like  is  true  of 
blood- corpuscles,  and  probably  of  many  other  elements  of 
the  tissues. 

This  form  of  local  death  is  insensible  to  ourselves,  and  is 
essential  to  the  due  maintenance  of  life.  But,  occasionally, 
local  death  occurs  on  a  larger  scale,  as  the  result  of  injury, 
or  as  the  consequence  of  disease.  A  burn,  for  example,  may 
suddenly  kill  more  or  less  of  the  skin  ;  or  part  of  the  tissues 
of  the  skin  may  die,  as  in  the  case  of  the  slough  which  lies 
in  the  midst  of  a  boil ;  or  a  whole  limb  may  die,  and  exhibit 
the  strange  phenomena  of  mortification. 

The  local  death  of  some  tissues  is  followed  by  their  regen- 
eration. Not  only  all  the  forms  of  epidermis  and  epithe- 
lium, but  nerves,  connective  tissue,  bone,  and  at  any  rate, 
some  muscles,  may  be  thus  reproduced,  even  on  a  large 
scale. 

(ii)  General  death  is  of  two  kinds,  death  of  the  body  as  a 
whole,  and  death  of  the  tissues.  By  the  former  term  is  im- 
plied the  absolute  cessation  of  the  functions  of  the  brain,  of 
the  circulatory,  and  of  the  respiratory  organs  ;  by  the  latter, 
the  entire  disappearance  of  the  vital  actions  of  the  ultimate 
structural  constituents  of  the  body.  When  death  takes  place, 
the  body,  as  a  whole,  dies  first,  the  death  of  the  tissues  not 


I  LIFE  AND   DEATH  27 

occurring  until  after  an  interval,  which  is  sometimes  consid- 
erable. 

Hence  it  is  that,  for  some  little  time  after  what  is  ordi- 
narily called  death,  the  muscles  of  an  executed  criminal 
may  be  made  to  contract  by  the  application  of  proper 
stimuli.     The  muscles  are  not  dead,  though  the  man  is. 

14.  Modes  of  Death.  —  The  modes  in  which  death  is 
brought  about  appear  at  first  sight  to  be  extremely  varied. 
We  speak  of  natural  death  by  old  age,  or  by  some  of  the 
endless  forms  of  disease  ;  of  violent  death  by  starvation,  or 
by  the  innumerable  varieties  of  injury,  or  poison.  But,  in 
reality,  the  immediate  cause  of  death  is  always  the  stoppage 
of  the  functions  of  one  of  three  organs  :  the  cerebro-spinal 
nervous  system,  the  lungs,  or  the  heart.  Thus,  a  man  may 
be  instantly  killed  by  such  an  injury  to  a  part  of  the  brain 
which  is  called  the  spinal  bulb  or  medulla  oblongata  (see 
p.  538)  as  may  be  produced  by  hanging,  or  breaking  the 
neck. 

Or  death  may  be  the  immediate  result  of  suffocation  by 
strangulation,  smothering,  or  drowning,  —  or,  in  other  words, 
of  stoppage  of  the  respiratory  functions. 

Or,  finally,  death  ensues  at  once  when  the  heart  ceases  to 
propel  blood.  These  three  organs  —  the  brain,  the  lungs, 
and  the  heart  —  have  been  fancifully  termed  the  tripod  of 
life. 

In  ultimate  analysis,  however,  life  has  but  two  legs  to 
stand  upon,  the  lungs  and  the  heart,  for  death  through 
the  brain  is  always  the  effect  of  the  secondary  action  of 
the  injury  to  that  organ  upon  the  lungs  or  the  heart.  The 
functions  of  the  brain  cease  when  either  respiration  or  cir- 
culation is  at  an  end.  But  if  circulation  and  respiration  be 
kept  up  artificially,  the  brain  may  be  removed  without  caus- 
ing death.    On  the  other  hand,  if  the  blood  be  not  aerated, 


28  ELEMENTARY   PHYSIOLOGY  less. 

its  circulation  by  the  heart  cannot  preserve  life ;  and,  if  the 
circulation  be  at  an  end,  mere  aeration  of  the  blood  in  the 
lungs  is  equally  ineffectual  for  the  prevention  of  death. 

15.  Decomposition  of  the  Body.  —  With  the  cessation  of 
life,  the  everyday  forces  of  the  inorganic  world  no  longer 
remain  the  servants  of  the  bodily  frame,  as  they  were  during 
life,  but  become  its  masters.  Oxygen,  the  slave  of  the  liv- 
ing organism,  becomes  the  lord  of  the  dead  body.  Atom 
by  atom,  the  complex  molecules  of  the  tissues  are  taken  to 
pieces  and  reduced  to  simpler  and  more  oxidised  substances, 
until  the  soft  parts  are  dissipated  chiefly  in  the  form  of  car- 
bonic acid,  ammonia,  water,  and  soluble  salts,  and  the  bones 
and  teeth  alone  remain.  But  not  even  these  dense  and  earthy 
structures  are  competent  to  offer  a  permanent  resistance  to 
water  and  air.  Sooner  or  later  the  animal  basis  which  holds 
together  the  earthy  salts  decomposes  and  dissolves  —  the 
solid  structures  become  friable,  and  break  down  into  powder. 
Finally,  they  dissolve  and  are  diffused  among  the  waters 
of  the  surface  of  the  globe,  just  as  the  gaseous  products  of 
decomposition  are  dissipated  through  its  atmosphere. 

It  is  impossible  to  follow,  with  any  degree  of  certainty, 
wanderings  more  varied  and  more  extensive  than  those 
imagined  by  the  ancient  sages  who  held  the  doctrine  of 
transmigration ;  but  the  chances  are,  that,  sooner  or  later, 
some,  if  not  all,  of  the  scattered  atoms  will  be  gathered  into 
new  forms  of  life. 

The  sun's  rays,  acting  through  the  vegetable  world,  build 
up  some  of  the  wandering  molecules  of  carbonic  acid,  of 
water,  of  ammonia,  and  of  salts,  into  the  fabric  of  plants. 
The  plants  are  devoured  by  animals,  animals  devour  one 
another,  man  devours  both  plants  and  other  animals ;  and 
hence  it  is  very  possible  that  atoms  which  once  formed  an 
integral  part  of  the  busy  brain  of  Julius  Caesar  may  now 


I  CHANGES  OF   MATTER  29 

enter  into  the  composition  of  Caesar  the  negro  in  Alabama, 
and  of  Caesar  the  house-dog  in  an  English  homestead. 

And  thus  there  is  sober  truth  in  the  words  which  Shake- 
speare puts  into  the  mouth  of  Hamlet  — 

"  Imperious  Caesar,  dead  and  turn'd  to  clay, 
Might  stop  a  hole  to  keep  the  wind  away : 
O,  that  that  earth,  which  kept  the  world  in  awe, 
Should  patch  a  wall  to  expel  the  winter's  flaw ! " 


LESSON    II 

THE   MINUTE    STRUCTURE   OF    THE   TISSUES 

1.  Every  Tissue  a  Compound  Structure.  —  In  the  first 
chapter  attention  was  directed  to  the  obvious  fact  that  the 
substance  of  which  the  body  of  a  man  or  other  of  the 
higher  animals  is  composed,  is  not  of  uniform  texture 
throughout ;  but  that,  on  the  contrary,  it  is  distinguishable 
into  a  variety  of  components,  which  differ  very  widely  from 
one  another,  not  only  in  their  general  appearance,  their 
colour,  and  their  hardness  or  softness,  but  also  in  their 
chemical  composition,  and  in  the  properties  which  they 
exhibit  in  the  living  state. 

In  dissecting  a  limb  there  is  no  difficulty  in  distinguish- 
ing the  bones,  the  cartilages,  the  muscles,  the  nerves,  and 
so  forth,  from  one  another ;  and  it  is  obvious  that  the  other 
limbs,  the  trunk,  and  the  head,  are  chiefly  made  up  of  simi- 
lar structures.  Hence,  when  the  foundations  of  anatomical 
science  were  laid,  more  than  two  thousand  years  ago,  these 
"like "  structures  which  occur  in  different  parts  of  the 
organism  were  termed  homoiomcra,  "  similar  parts."  In 
modern  times  they  have  been  termed  tissues,  and  the 
branch  of  biology  which  is  concerned  with  the  investigation 
of  the  nature  of  these  tissues  is  called  Histology. 

Histology  is  a  very  large  and  difficult  subject,  and  this 
whole  book  might  well  be  taken  up  with  a  thorough  dis- 
cussion of  even  its  elements.  But  physiology  is,  in  ultimate 
analysis,  the  investigation  of  the  vital  properties  of  the  his- 

3° 


less,  ii  THE  TISSUES  31 

tulogical  units  of  which  the  body  is  composed.  And  even 
the  elements  of  physiology  cannot  be  thoroughly  compre- 
hended without  a  clear  apprehension  of  the  nature  and 
properties  of  the  principal  tissues. 

A  good  deal  may  be  learned  about  the  tissues  without 
other  aid  than  that  of  the  ordinary  methods  of  anatomy, 
and  it  is  extremely  desirable  that  the  student  should  acquire 
this  knowledge  as  a  preliminary  to  further  inquiry.  But  the 
chief  part  of  modern  histology  is  the  product  of  the  appli- 
cation of  the  microscope  to  the  elucidation  of  the  minute 
structure  of  the  tissues ;  and  this  has  had  the  remarkable 
result  of  proving  that  these  tissues  themselves  are  made 
up  of  extremely  small  homoiomera,  or  similar  parts,  which 
are  primitively  alike  in  form  in  all  the  tissues. 

Every  tissue  therefore  is  a  compound  structure  :  a  mul- 
tiple of  histological  units,  or  an  aggregation  of  histological 
elements ;  and  the  properties  of  the  tissue  are  the  sum  of 
the  properties  of  its  components.  The  distinctive  charac- 
ter of  every  fully- formed  tissue  depends  on  the  structure, 
mode  of  union,  and  vital  properties  of  its  histological  ele- 
ments when  they  are  fully  formed. 

2.  The  Embryonic  Tissues  and  the  Cells.  Protoplasm. 
—  Each  tissue  can  be  traced  back  to  a  young  or  embryonic 
condition,  in  which  it  has  no  characteristic  properties,  and 
in  which  its  histological  elements  are  so  similar  in  structure, 
mode  of  union,  and  vital  properties  to  those  of  every  other 
embryonic  tissue,  that  our  present  means  of  investigation 
do  not  enable  us  to  discover  any  difference  among  them. 

These  embryonic,  undifferentiated,  histological  elements, 
of  which  every  tissue  is  primitively  composed,  or,  as  it 
would  be  more  correct  to  say,  which,  in  the  embryonic 
condition,  occupy  the  place  of  the  tissues,  are  technically 
named  cells.     The  colourless  blood-corpuscle   (p.   126)   is 


32  ELEMENTARY   PHYSIOLOGY  les:. 

a  typical  representative  of  such  a  cell.  And  it  is  substan- 
tially correct  to  say  (i)  that  the  histological  elements  of 
every  tissue  are  modifications  or  products  of  such  cells ; 
(2)  that  every  tissue  was  once  a  mass  of  such  cells  more 
or  less  closely  packed  together;  and  (3)  that  the  whole 
embryonic  body  was  at  one  time  nothing  but  an  aggrega- 
tion of  such  cells.  In  its  undifferentiated  condition,  each 
cell  consists  mainly  of  a  soft,  colourless  mass  of  living  sub- 
stance, in  consistency  somewhat  thicker  than  raw  white  of 
egg,  and  containing  more  or  less  non-living  material.  There 
is  imbedded  in  it  a  somewhat  denser  body,  also  mainly 
living,  termed  the  nucleus.  The  living  substance,  whether 
occurring  in  the  body  of  the  cell  or  in  the  nucleus,  is  called 
protoplasm.  This  substance  is  the  material  basis  of  life 
wherever  life  occurs,  whether  in  the  human  body,  in  the 
bodies  of  lower  animals,  or  in  plants. 

Besides  the  living  cells,  every  tissue  contains  a  greater 
or  less  quantity  of  lifeless  substance,  lying  among  the  cells, 
and    hence   called   intercellular   sub- 
stance.     This   is   produced   at  some 
time  by  the  living  cells.      As  will  be 
seen,  it  varies  greatly  in  quantity  and 
characteristics  in  the  different  tissues. 
3.   The  Body  starts  as    a   Single 
Cell,  the  Ovum,  which  then  divides 
FlG'     _IoCumAM  °F  ™E   into  Primitive  Cells.  — The  body  of 
a,  granular  protoplasm;    a  man  or  of  any  of   the   higher  ani- 
ves?de""Sc,Cnucieoi'uff  raffed    mals  commences  as  an  ovum  or  egg. 

"germinal  spot."  Thig    ^  pig_   ^    ;g  R   minute   spheroidal 

body  200  fx  (T|-g-  of  an  inch)  in  diameter  in  man,  consisting 
of  protoplasm,  in  which  a  single  large  nucleus  is  imbedded, 
and  covered  by  a  transparent  membrane. 

The  first  step    towards  the  production  of  all    the  com- 


ii  THE  TISSUES  33 

plex  organisation  of  a  mammal  out  of  this  simple  budy  is 
the  division  of  the  nucleus  into  two  new  nuclei,  which  re- 
cede from  one  another,  while  at  the  same  time  the  proto- 
plasmic body  becomes  divided,  by  a  narrow  cleft  which 
runs  between  the  two  nuclei,  into  two  masses,  or  blasto- 
meres (Fig.  8,  a),  one  for  each  nucleus.  By  the  repetition 
of  the  process  the  two  blastomeres  give  rise  to  four,  the 
four  to  eight,  the  eight  to  sixteen,  and  so  on,  until  the 
embryo  is  an  aggregate  of  numerous  small  blastomeres,  or 
nucleated  cells.  These  grow  at  the  expense  of  the  nutri- 
ment supplied  from  without,  and  continue  to  multiply 
by  division  according  to  the  tendencies  inherent  in  each 
until,  long  before  any  definite  tissue  has  made  its  appear- 
ance, they  build  themselves  up  into  a  kind  of  sketch 
model  of  the  developing  animal,  in  which  model  many  of, 
if  not  all,  the  future  organs  are  represented  by  mere  aggre- 
gates of  undifferentiated  cells. 

4.  The  Differentiation  of  the  Primitive  Cells.  — 
Gradually,  these  undifferentiated  cells  become  changed, 
as  regards  their  shape,  size,  and  structure,  into  groups  or 
sets  of  differentiated  cells,  the  cells  in  one  set  being  like 
each  other,  but  unlike  those  of  other  sets.  Each  set  of 
differentiated  cells  constitutes  a  "  tissue,"  and  each  tissue 
is  variously  distributed  among  the  several  organs,  each 
organ  generally  consisting  of  more  than  one  tissue. 

And  this  differentiation  of  form  is  accompanied  by  a 
change  of  properties.  The  undifferentiated  cells  are,  as 
far  as  we  can  see,  alike  in  function  and  properties  as  they 
are  alike  in  form.  But  coincident  with  their  differentiation 
into  tissues,  a  division  of  labour  takes  place,  so  that  in  one 
tissue  the  cells  manifest  special  properties  and  carry  on 
a  special  work  ;  in  another  they  have  other  properties,  and 
other  work  ;  and  so  on. 

D 


34 


ELEMENTARY    PHYSIOLOGY 


5.  The  Chief  Tissues  of  the  Body. — The  principal 
tissues  into  which  the  undifferentiated  cells  of  the  embryo 
become  differentiated,  and  which  are  variously  built  up  into 
the  organs  and  parts  of  the  adult  body,  may  be  arranged  as 
follows. 


Fig.  8.  —  The   Successive  Division  of  the  Mammalian   Ovum    into   Blasto- 
meres.     Somewhat  diagrammatic. 

a,  division  into  two,  b,  into  four,  c,  into  eight,  and  d,  into  many  blastomeres. 
The  clear  ring  seen  in  each  case  is  the  zona  pellucida,  or  membrane  investing  the 
ovum. 


(i)  The  most  important  tissues  are  the  muscular  and  ner- 
vous tissues,  for  it  is  by  these  that  the  active  life  of  the 
individual  is  carried  on. 

(ii)  Next  come  the  epithelial  tissues,  which,  on  the  one 
hand,  afford  a  covering  for  the  surface  of  the  body  as  well  as 
a  lining  for  the  various  internal  cavities,  and,  on  the  other 
hand,  carry  on  a  great  deal  of  the  chemical  work  of  the 
body,  inasmuch  as  they  form  the  essential  part  of  the  various 
glandular  organs. 


ii  THE   EPIDERMIS  35 

(iii)  The  remaining  principal  tissues  of  the  body,  namely 
the  so-called  connective  tissue,  cartilaginous  tissue,  and 
osseous  or  bony  tissue,  form  a  group  by  themselves,  being 
all  three  similar  in  their  fundamental  structure,  and  all  three 
being,  for  the  most  part,  of  use  to  the  body  for  iheir  passive 
rather  than  for  their  active  qualities.  They  chiefly  serve  to 
support  and  connect  the  other  tissues. 

These  principal  or  fundamental  tissues  are  often  arranged 
together  to  form  more  complex  parts  of  the  body,  which  are 
sometimes  spoken  of,  though  in  a  different  sense,  as  tissues. 
Thus,  various  forms  of  connective  tissue  are  built  up,  with 
some  muscular  tissue  and  nervous  tissue,  to  form  the  blood- 
vessels (see  Lesson  III.),  which  are  sometimes  spoken  of  as 
"vascular  tissue."  So,  again,  a  certain  kind  of  epithelial 
tissue,  known  as  "  epidermis,"  together  with  connective 
tissue,  blood-vessels,  and  nerves,  forms  the  skin  or  tegument- 
ary  tissue  ;  a  similar  combination  of  epithelium  with  other 
tissues  constitutes  the  mucous  membrane  lining  the  ali- 
mentary canal,  and  also  occurs  in  the  so-called  "glandular" 
tissue.  The  structure  of  these,  as  also  of  muscle  and  nerve 
and  bone,  will  be  described  later,  and  we  may  confine  our 
attention  here  to  the  other  principal  tissues :  epithelial 
tissues,  the  connective  tissues,  and   cartilage. 

6.  The  Structure  of  the  Epidermis.  —  A  good  example 
of  this  tissue  is  to  be  found  in  the  skin,  which  consists  of  the 
superficial  epidermis,  which  is  non-vascular  and  epithelial  in 
nature,  and  of  the  deep  dermis,  which  is  vascular,  and  is,  in- 
deed, chiefly  composed  of  connective  tissue  carrying  blood- 
vessels and  nerves  (Fig.  65,  p.  216).  And  in  all  the  mucous 
membranes  there  is  a  similar  superficial  epithelial  layer, 
which  is  there  simply  called  epithelium,  and  a  deep  layer, 
which  similarly  consists  of  connective  tissue  carrying  blood 
vessels  and  nerves  and  may  also  be  spoken  of  as  dermis. 


36  ELEMENTARY    PHYSIOLOGY  less 

If  a  piece  of  fresh  skin  is  macerated  for  some  time  in 
water,  it  is  easy  to  strip  off  the  epidermis  from  the  dermis. 

The  outer  part  of  the  epidermis  which  has  been  detached 
by  maceration  will  be  found  to  be  tolerably  dense  and  cohe- 
rent, while  its  deep  or  inner  substance  is  soft  and  almost 
gelatinous.  Moreover,  this  softer  substance  fills  up  all  the 
irregularities  of  the  surface  of  the  dermis  to  which  it  adheres, 
and  hence,  where  the  dermis  is  raised  up  into  papilla?,  the 
deep  or  under  surface  of  the  epidermis  presents  innumer- 
able depressions,  into  which  the  papilla?  fit,  giving  it  an 
irregular  appearance,  somewhat  like  a  network.  Hence  it 
used  not  unfrequently  to  be  called  the  network  of  Malpighi 
(rete  Malpighii) ,  after  a  great  Italian  anatomist  of  the  sev- 
enteenth century,  who  first  properly  described  it.  On  the 
other  hand,  its  soft  and  gelatinous  character  led  to  its  being 
called  mucous  layer  {stratum  mucosuni).  Its  common 
name  is  Malpighian  layer.  Chemical  analysis  shows  that 
the  firm  outer  layer  of  the  epidermis  differs  from  the 
deep  soft  part  by  containing  a  great  deal  of  horny  matter. 
Hence  this  is  distinguished  as  the  horny  layer  {stratum 
corneum). 

In  the  living  subject  the  superficial  layers  of  the  epi- 
dermis become  separated  from  the  lower  layers  and 
the  dermis,  when  friction  or  other  irritation  produces  a 
"  blister."  Fluid  is  poured  out  from  the  vessels  of  the 
dermis,  and,  accumulating  between  the  upper  and  lower 
layers  of  the  epidermis,  detaches  the  former. 

The  epidermis  is  constantly  growing  upon  the  deep  or 
dermal  side  in  such  a  manner  that  the  horny  layer  is  con- 
tinually being  shed  and  replaced.  The  "scurf"  which 
collects  between  the  hairs  and  on  the  whole  surface  of  the 
body,  and  is  removed  by  our  daily  brushing  and  washing, 
is  nothing  but  shed  epidermis.     When  a  limb   has   been 


II  THE   EPIDERMIS  37 

bandaged  up  and  left  undisturbed  for  weeks,  as  in  case  of 
a  fracture,  the  shed  epidermis  collects  on  the  surface  of  the 
skin  in  the  shape  of  scales  and  flakes,  which  break  up  into 
a  fine  white  powder  when  rubbed.  Thus  we  "  shed  our 
skins  "  just  as  snakes  do,  only  that  the  snake  sheds  all  his 
dead  epidermis  as  a  coherent  sheet  at  once,  while  we  shed 
ours  bit  by  bit,  and  hour  by  hour. 

What  is  the  nature  of  the  process  by  which  the  epidermis 
is  continually  removed  ? 

If  a  little  of  the  epidermal  scurf  is  mixed  with  water  and 
examined  under  a  power  magnifying  300  or  400  diameters,  it 
will  seem  to  consist  of  nothing  but  irregular  particles  of  very 
various  sizes  and  with  no  definite  structure.  But  if  a  little 
caustic  potash  or  soda  is  previously  added  to  the  water,  the 
appearance  will  be  changed.  The  caustic  alkali  causes  the 
horny  substance  to  swell  up  and  become  transparent ;  and 
this  is  now  seen  to  consist  of  minute  separable  plates,  some 
of  which  contain  a  rounded  body  in  the  interior  of  the  plate, 
though  in  many  this  is  no  longer  recognisable.  In  fact,  so 
far  as  their  form  is  concerned,  these  bodies  have  the  char- 
acter of  nucleated  cells,  in  which  the  protoplasmic  body 
has  been  more  or  less  extensively  converted  into  horny 
substance. 

Thus  the  cast-off  epidermis  in  reality  consists  of  more  or 
less  coherent  masses  of  cornified  nucleated  cells. 

There  is  a  yet  simpler  method  of  demonstrating  this  truth. 
At  the  margins  of  the  lips  the  epidermis  is  continued  into 
the  interior  of  the  mouth,  and,  though  it  now  receives  the 
name  of  epithelium,  it  differs  from  the  rest  of  the  skin  in  no 
essential  respect  except  that  it  is  very  thin,  and  allows  the 
blood  in  the  vessels  of  the  subjacent  dermis  to  shine  through. 
Let  the  lower  lip  be  turned  down,  its  surface  very  gently 
scraped  with  a  blunt-edged  knife,  and  the  substance  removed 


38  ELEMENTARY   PHYSIOLOGY  less. 

be  spread  out,  covered  with  a  thin  glass,  and  examined  as 
before.  The  whole  field  of  view  will  then  be  seen  to  be 
spread  over  with  flat  irregular  bodies  very  like  the  epidermal 
scales,  but  more  transparent,  and  each  provided  with  a 
nucleus  in  its  centre  (Fig.  9). 


Fig.  9. — Two  Epithelial  Scales  from  the  Interior  of  the  Mouth. 

A  small  nucleus  n  is  seen  in  each,  as  well  as  fine  granulations  in  the  body  of  the 
cell.  The  edges  of  the  cells  are  irregular  from  pressure.  Magnified  about  400 
times. 

Since  these  detached  scales  are  always  to  be  found  on  the 
inner  surface  of  the  lip,  it  follows  that  they  are  always  being 
thrown  off. 

7.  The  Growth  of  the  Epidermis.  —  The  horny  external 
layer  of  the  epidermis  is  composed  of  coherent  cornified 
flattened  cells,  which  are  constantly  becoming  detached 
from  the  soft  internal  layer,  and  must  needs  be,  in  some 
way,  derived  from  it.  But  in  what  way  ?  Here  microscopic 
investigation  furnishes  the  answer.  For,  if  the  soft  layer  is 
properly  macerated,  it  breaks  up  into  small  masses  of  nucle- 
ated protoplasmic  substance,  that  is,  into  nucleated  cells, 
which  in  the  innermost  or  deepest  part  of  the  layer  are  colum- 
nar in  form,  being  elongated  perpendicularly  to  the  face  of 
the  dermis,  on  which  they  rest,  and  which  in  the  interme- 
diate region  present  transitions  in  form  and  othei  respects 
between  these  and  the  shed  scales. 


II  GROWTH   OF   EPIDERMIS  39 

A  thin  vertical  section  of  epidermis  (see  Fig.  65,  p.  216), 
in  undisturbed  relation  with  the  subjacent  dermis,  leaves  not 
the  smallest  doubt  (a)  that  the  epidermis  consists  of  noth- 
ing but  nucleated  cells,  with  perhaps  an  infinitesimal  amount 
of  cementing  substance  between  them ;  {b)  that,  from  the 
deep  to  the  superficial  part  of  the  epidermis,  the  cells  always 
present  a  succession  from  columnar  or  subcylindrical,  proto- 
plasmic forms  to  flattened,  completely  cornified  forms.  And, 
since  we  know  that  the  latter  are  constantly  being  thrown 
off,  it  follows  (Y)  that  these  gradations  of  form  represent 
cells  of  the  deep  layer  which  are  continually  passing  to  the 
surface  and  there  being  thrown  off. 

What  is  the  cause  of  this  constant  succession  ?  To  this 
question,  also,  microscopic  investigation  furnishes  a  clear 
answer.  The  deeper  cells  are  constantly  growing  and  then 
multiplying  by  a  process  of  division  in  such  a  manner  that 
the  nucleus  of  a  cell  divides  into  two  new  nuclei,  around 
each  of  which  one-half  of  the  protoplasmic  body  disposes 
itself.  Thus  one  cell  becomes  two,  and  each  of  these  grows 
until  it  acquires  its  full  size  at  the  expense  of  the  nutritive 
matters  which  exude  from  the  vessels  with  which  the  dermis 
is  abundantly  supplied  ;  such  a  cell,  in  fact,  possesses  the 
vital  properties  of  a  primitive  embryonic  cell. 

The  cells  nearer  the  dermis  are  more  immediately  and 
abundantly  supplied  with  nourishment  from  the  dermal 
blood-vessels,  and  serve  as  the  focus  of  growth  and  multi- 
plication for  the  whole  epidermis  ;  they  are,  in  fact,  the  pro- 
genitors of  the  superficial  cells,  which,  as  they  are  thrust 
away  by  the  intercalation  of  new  cells  between  the  last 
formed  and  the  progenitors,  become  metamorphosed  in 
form  and  chemical  character,  and  at  last  die  and  are  cast  off. 

And  it  follows  that  the  epidermis  is  to  be  regarded  as  a 
compound  organism  made  up  of  myriads  of  cells,  each  of 


40  ELEMENTARY   PHYSIOLOGY  less. 

which  follows  its  own  laws  of  growth  and  multiplication,  and 
is  dependent  upon  nothing  save  the  due  supply  of  nutriment 
from  the  dermal  vessels.  The  epidermis,  so  far,  stands  in 
the  same  relation  to  the  dermis  as  does  the  turf  of  a  meadow 
to  the  subjacent  soil. 

8.  The  Unit  used  in  Histological  Measurement. — 
Structures  which  are  rendered  clearly  distinguishable  only 
by  a  magnifying  power  of  300  or  400  diameters  must  needs 
be  very  small,  and  it  is  desirable  that,  before  going  any  fur- 
ther, the  learner  should  try  to  form  a  definite  notion  of  their 
actual  and  relative  dimensions  by  comparison  with  more 
familiar  objects.  A  hair  of  the  human  head  of  ordinary 
fineness  has  a  diameter  of  about  ¥^~q  (say  0-003)  of  an  inch, 
or  0-08  mm.  (millimetre).  The  hairs  which  constitute  the 
fur  of  a  rabbit,  on  the  other  hand,  are  very  much  finer,  and 
the  thickest  part  of  the  shaft  usually  does  not  exceed  10100 
of  an  inch,  i.e.  o-ooi  inch,  or  about  0-025  mnl-  J  while  the 
fine  point  of  such  a  hair  may  be  as  little  as  sjinru  °^  an 
inch,  about  o-ooi  mm.,  or  even  less,  in  diameter. 

In  microscopic  histological  investigations  the  range  of 
the  magnitudes  with  which  we  have  to  deal  ordinarily  lies 
between  o-i  and  o-ooi  millimetre;  that  is  to  say,  roughly, 
between  one  two  hundred  and  fiftieth  and  one  twenty-five 
thousandth  of  an  inch.  It  is,  therefore,  extremely  conven- 
ient to  adopt,  as  a  unit  of  measurement,  o-ooi  millimetre, 
called  a  micro-millimetre,  and  indicated  by  the  symbol  /*, 
of  which  all  greater  magnitudes  are  multiples.1  Thus,  if  the 
extreme  point  of  a  rabbit's  hair  has  a  diameter  of  i/x,  the 
middle  of  the  shaft  will  be  25^,  and  the  shaft  of  a  hair  of 
the  human  head  80  fx. 

Adopting  this  system,  the  deep  cells  of  epidermis  have  on 

1  Since  1  millimetre  is  very  nearly  equal  to  jfe  of  an  inch,  ^  =  jeJoo  of  an 
inch. 


a  EPITHELIUM  41 

an  average  a  diameter  of  12/x  or  more,  the  nuclei  of  4/^  to 
5jh,  while  the  superficial  cells  are  plates  of  about  25/A,  the 
nuclei  retaining  about  the  same  dimensions. 

9.  The  Epithelium  of  Mucous  Membrane.  —  The  mucous 
membrane  lining  the  alimentary  canal,  as  has  been  stated,  is 
framed  on  the  plan  of  the  skin,  inasmuch  as  it  consists  of  a 
vascular  dermis,  and  a  non-vascular  epithelium,  the  latter 
being  composed  of  cells  in  juxtaposition.  But,  except  in 
the  region  of  the  mouth,  where  the  epithelium,  like  the 
epidermis,  is  composed  of  many  layers  of  cells,  arranged  as 
a  soft  Malpighian  layer  and  a  horny  layer,  and  the  oesopha- 
gus, where  the  structure  is  similar,  the  epithelium  of  the  ali- 
mentary canal  and  the  continuations  of  that  epithelium  into 
the  various  glands,  such  as  the  salivary  glands,  glands  of 
the  stomach,  the  pancreas,  the  liver,  etc.,  consist  of  hardly 
more  than  a  single  layer  of  cells  placed  side  by  side. 
Hence  in  a  vertical  section  of  the  mucous  membrane  the 
vascular  part  is  seen  to  be  covered  by  a  single  row  of  soft, 
nucleated  cells ;  though  sometimes  a  second  row  of  incon- 
spicuous small  cells  may  be  seen  below  the  latter.  The 
cells  constituting  this  single  layer  vary  in  shape,  being  cylin- 
drical or  conical  or,  as  especially  in  the  glands,  cubical  or 
sphenoidal ;  but  they  always  are  delicate  masses  of  proto- 
plasm, each  containing  a  nucleus. 

In  the  air  passages  of  the  lungs  and  in  certain  other  places 
the  epithelium  of  the  mucous  membrane  consists  again  of 
several  layers  of  cells,  but  all  are  soft  and  protoplasmic  nucle- 
ated masses,  the  uppermost  layer  being  cylindrical  in  form. 
The  exposed  ends  of  the  cells  in  the  uppermost  layer  are  cov- 
ered by  innumerable  minute,  hair-like  projections  from  the 
body  of  the  cells,  like  the  nap  of  velvet,  and  called  cilia ; 
such  epithelium  is  called  ciliated  epithelium  (see  Fig.  90, 
p.  308).      During   life    the   cilia   are    in   constant    waving 


42  ELEMENTARY   PHYSIOLOGY  less. 

motion,  sweeping  along  foreign  matter  that  comes  in  contact 
vith  them,  and  thus  protecting  the  cells. 

Lastly,  the  blood-vessels  and  lymphatic  vessels  and  the 
large  cavities,  such  as  the  chest  and  abdomen,  are  lined  by 
a  peculiar  epithelium,  different  in  origin  from  the  epithelium 
of  the  skin  and  mucous  membranes.  It  consists  of  a  single 
layer  of  flat,  nucleated  plates,  cemented  together  at  their 
edges.  The  form  of  the  plate  or  cell  varies,  being  some- 
times polygonal,  sometimes  spindle-shaped,  sometimes  quite 
irregular. 

10.  The  Structure  of  Cartilage.  —  A  second  group  of 
tissues,  of  which  cartilage  may  be  taken  as  the  simplest  form 
and  the  type,  differs  from  epithelium  in  a  very  essential 
feature.  In  epithelium,  wherever  it  is  found,  the  cells  are 
placed  close  together,  and  the  amount  of  material  existing 
between  the  cells,  or  intercellular  material,  is  exceedingly 
small.  In  the  group  of  tissues,  however,  to  which  cartilage 
belongs,  a  very  considerable  quantity  of  intercellular  mate- 
rial is,  as  we  shall  see,  developed  between  the  individual 
nucleated  protoplasmic  cells.  Hence  the  cells  are,  more  or 
less,  distinctly  imbedded  in  a  substance  different  from  them- 
selves and  called  a  matrix.  In  epithelium,  though  the  cells 
are  sometimes  joined  together  by  a  cement  material,  this  is 
never  abundant  enough  to  deserve  the  name  of  matrix. 

(i)  Hyaline  Cartilage.  —  Characteristic  specimens  of  this 
tissue  are  to  be  found  in  the  "  sterno-costal  cartilages," 
which  unite  many  of  the  ribs  with  the  breast-bone.  A  thin 
but  tough  layer  of  vascular  connective  tissue  invests,  and 
closely  adheres  to,  the  surface  of  the  cartilage.  It  is  termed 
the  perichondrium.  The  substance  of  the  cartilage  itself  is 
devoid  of  vessels;  it  is  hard,  but  not  very  brittle,  for  it  will 
bend  under  pressure  ;  and,  moreover,  it  is  elastic,  returning 
to  its  original  shape  when  the  pressure  is  removed.     It  may 


II  CARTILAGE  43 

easily  be  cut  into  very  thin  slices,  which  are  as  transparent 
as  glass,  and  to  the  naked  eye  appear  homogeneous.  Dilute 
acids  and  alkalies  have  no  effect  upon  it  in  the  cold ;  but,  if 
it  is  boiled  in  water,  it  yields  a  substance  similar  to  gelatin 
but  somewhat  different  from  it,  which  is  called  chondrin. 

The  sterno-costal  cartilages  of  an  adult  man  are  many 
times  larger  than  those  of  an  infant.  It  follows  that  these 
cartilages  must  grow.  The  only  source  from  whence  they 
can  derive  the  necessary  nutritive  material  is  the  plasma 
exuded  from  the  blood  contained  in  the  vessels  of  the  peri- 
chondrium. The  vascular  perichondrium,  therefore,  stands 
in  the  same  relation  to  the  non-vascular  cartilaginous  tissue 
as  the  vascular  dermis  does  to  the  non-vascular  epidermis. 
But,  since  the  cartilage  is  invested  on  all  sides  by  the  peri- 
chondrium, it  is  clear  that  no  part  of  the  cartilage  can  be 
shed  in  the  fashion  that  the  superficial  layers  of  epidermis 
are  got  rid  of.  As  the  nutritive  materials,  at  the  expense  of 
which  the  cartilage  grows,  are  supplied  from  the  perichon- 
drium, it  might  be  concluded  that  the  cartilage  grows  only 
at  its  surface.  But,  if  a  piece  of  cartilage  is  placed  in  a 
staining  fluid,  it  will  be  found  that  it  soon  becomes  more  or 
less  coloured  throughout.  In  spite  of  its  density,  therefore, 
cartilage  is  very  permeable,  and  hence  the  nutritive  plasma 
also  may  permeate  it,  and  enable  every  part  to  grow. 

If  a  thin  section  of  perfectly  fresh  and  living  cartilage  is 
placed  on  a  glass  slide,  either  without  addition  or  with  only 
a  little  serum,  it  appears  to  the  naked  eye,  as  has  been  said, 
to  be  as  homogeneous  as  a  piece  of  glass.  But  the  employ- 
ment of  an  ordinary  hand  magnifier  is  sufficient  to  show  that 
it  is  not  really  homogeneous,  inasmuch  as  minute  points  of 
less  transparency  are  seen  to  be  scattered  singly  or  in  groups 
throughout  the  thickness  of  the  section.  When  the  section 
is  examined  with  the   microscope   (Fig.    10)   these   points 


44  ELEMENTARY   PHYSIOLOGY  less* 

prove  to  be  nucleated  cells,  the  cartilage  corpuscles,  vary, 
ing  in  shape,  but  generally  more  or  less  spheroidal,  some- 
times far  apart,  sometimes  very  near,  or  in  groups  in  contact 
with  one  another,  in  which  last  case  the  applied  sides  are  flat. 
Usually  each  cell  has  a  single  nucleus,  but  sometimes  there 
are  two  nuclei  in  a  cell.  And  sometimes  globules  of  fat 
appear  in  the  protoplasmic  bodies  of  the  cells,  and  may 
completely  fill  them. 


&  :  I 


-/S:     V 


Fig.  io.  —  Hyaline  Cartilage.    A  Thin  Section  highly  Magnified. 

m,  matrix;  a,  a  group  of  two  cartilage  cells;  b,  a  group  of  four  cells;  c,  a  cell; 
n,  nucleus. 

As  a  rule  each  cell  lies  in,  and  exactly  fills,  a  cavity  in 
the  transparent  matrix,  or  intercellular  substance,  which 
constitutes  the  chief  mass  of  the  tissue.  But  a  pair  of 
closely  opposed  flattened  cells  may  occupy  only  one  cavity, 
and  all  sorts  of  gradations  may  be  found  between  hemi- 
spheroidal  cells  in  contact,  and  hemi-spheroidal  cells  sepa- 
rated by  a  mere  film  of  intercellular  substance,  and  widely 
separate  spheroidal,  ellipsoidal,  or  otherwise  shaped  cells. 
In  size,  the  cells  vary  very  much,  some  being  as  small  as 
io/a,  and  others  as  large  as  50/x.,  or  even  larger. 

As  the  cartilage  dies,  and  especially  if  water  is  added  to 
it,  the  protoplasmic  bodies  of  the  cells  shrink  and  become 
irregularly  drawn  away  from  the  walls  of  the  cavities  which 


CARTILAGE 


45 


contain  them,  and  the  appearance  of  the  tissue  is  greatly 
altered. 

No  structure  is  discernible  in  the  matrix  or  intercellular 
substance  under  ordinary  circumstances ;  but  it  may  be 
split  up  into  thin  sheets  or  laminae.  The  portions  of  matrix 
immediately  surrounding  the  several  cavities  sometimes 
differ  in  appearance  and  nature  from  the  rest  of  the  matrix, 
so  as  to  constitute   distinct  capsules   (Fig.  ii,  r)   for  the 


Fig.  ii.  —  A  Small  Portion  of  a  Section  of  Articular  Cartilage  (Frog),  verv 
highly  magnified  (6oodiam.). 

j,  matrix  or  intercellular  substance;  /,  the  protoplasmic  body  of  a  cartilage  cor- 
puscle; n,  its  nucleus;  «',  nucleoli;  c,  the  capsule,  or  wall  of  the  cavity  in  which 
the  cartilage  corpuscle  lies.  The  four  cells  here  figured  seem  to  have  arisen  from  a 
single  cell,  by  division,  first  into  two  and  then  into  four.  The  shading  of  the  matrix 
in  an  oblique  line  indicates  the  earlier  division  into  two. 


cells  ;  and,  at  times,  the  matrix  may  by  appropriate  methods 
be  split  up  into  pieces,  each  belonging  to  and  surrounding 
a  cell,  or  group  of  cells,  and  often  disposed  in  concentric 
layers. 

Close  to  the  perichondrial   surface  of  the  cartilage   the 
cells    become   smaller  and   separated  by  less  intercellular 


46 


ELEMENTARY   PHYSIOLOGY 


substance,  until  at  length  the  transparent  chondrigenous 
material  is  replaced  by  the  fibrous  collagenous  substance  of 
connective  tissue  (p.  51),  and  the  cartilage  cells  take  on 
the  form  of  "  connective  tissue  corpuscles." 

(ii)  White  Fibro-cartilage.  —  Since  cartilage  is  a  tissue 
which  serves  chiefly  for  the  purposes  of  supporting  and  con- 
necting other  structures  of  the  body,  it  requires,  in  certain 
positions,  to  be  somewhat  more  tough  and  resistant,  less 
brittle  and  more  flexible,  than  in  others.  Thus,  in  some 
joints,  as,  for  instance,  the  knee,  there  are  little  pads  or 
discs  of  cartilage  between  the  ordinary  articular  cartilage 


rr^ry^- 


Fig.  12.  -  Section  of  White  Fibro  Cartilage.     (Hardy.) 

(see  Fig.  104,  c).  Similar  discs  lie  in  between  and  are  at- 
tached to  the  bodies  of  the  vertebrae.  They  act  not  only 
as  a  sort  of  cushion  to  break  the  "jar"  arising  from  a  sud- 
den concussion  of  the  vertebral  column,  but  also  bind  the 
vertebrae  into  a  column  which  is  resistant  but  at  the  same 
time  flexible.  The  additional  strength  required  by  the  car- 
tilages of  these  discs  is  provided  by  the  introduction  into 
their  matrix  of  bundles  of  white  fibrous  connective  tissue ; 
hence  the  name,  white  fibro-cartilage  (Fig.  12). 

(iii)  Yellow  or  Elastic  Fibro-cartilage.  —  In  certain  other 
parts  of  the  body  cartilage  is  required  to  be  peculiarly 
elastic  and  flexible,  as  in  the  epiglottis  and  cartilage  of  the 
external  ear.     In  this  case  the  requisite  elasticity  is  given  to 


GROWTH    OF   CARTILAGE 


47 


it  by  the  introduction  into  the  matrix  of  a  dense  feltwork  of 
fibres  of  yellow  or  elastic  connective  tissue  (Fig.  .13). 

11.  The  Development  of  Cartilage.  —  In  a  very  young 
embryo  we  find  in  the  place  of  a  sterno-costal  cartilage  noth- 
ing but  a  mass  of  closely  applied,  undifferentiated,  nucleated 
cells,  having  the  same  essential  characters  as  the  deepest 
epidermal  cells.  The  rudiment,  or  embryonic  model  of 
the  future  cartilage  thus  constituted,  increases  in  size  by 
the  growth  and  division  of  the  cells.  But,  after  a  time,  the 
characteristic  intercellular  substance  appears,  at  first  in  small 


Fig.  13. —  Section  of  Yellow  Elastic  Cartilage.     (Hardy.) 

quantity,  between  the  central  cells  of  the  mass,  and  a  deli- 
cate sterno-costal  cartilage  is  thus  formed.  This  is  con- 
verted into  the  full-grown  cartilage  (a)  by  the  continual 
division  and  subsequent  growth  to  full  size  of  all  its  cells, 
and  especially  of  those  which  lie  at  the  surface  ;  (/>)  by  the 
constant  increase  in  the  quantity  of  intercellular  substance, 
especially  in  the  case  of  the  deeper  part  of  the  cartilage. 

The  manner  in  which  this  intercellular  substance  is  in- 
creased is  not  certainly  rriade  out.  If  the  outermost  layer 
only  of  each  of  the  protoplasmic  bodies  of  adjacent  cells  of 
the  epidermis  were  to  become  cornified  and  fused  together 
into  one  mass,  while  the  remainder  of  each  cell  continued 
to  grow  and  divide  and  its  progeny  threw  off  fresh  outer 


48  ELEMENTARY   PHYSIOLOGY  less. 

cornified  layers,  we  should  have  an  epidermal  structure 
which  would  resemble  cartilage  except  that  the  "intercellular 
substance  "  would  be  corneous  and  not  chondrigenous.  And 
it  is  possible  that  the  intercellular  substance  of  cartilage  may 
be  formed  in  this  way.  But  it  is  possible  that  the  chondrig- 
enous material  may  be,  as  it  were,  secreted  by  and  thrown 
out  between  the  cells,  as  we  shall  find  the  cells  of  glands  to 
secrete  the  gland  products,  or  at  all  events  manufactured 
in  some  way  by  the  agency  of  the  cells,  without  the  sub- 
stance of  the  cells  being  actually  transformed  into  it.  Thus, 
the  capsule,  of  each  cell  may  be  such  a  secretion,  which 
then  fuses  into  the  adjacent  matrix.  Our  knowledge  will 
not  at  present  permit  us  to  form  a  definite  judgment  on  this 
point.  One  thing,  however,  seems  certain,  viz.,  that  the 
cells  are  in  some  way  concerned  in  the  matter ;  the  matrix 
is  unable  to  increase  itself  in  the  entire  absence  of  cells. 

The  embryonic  cells  which  give  rise  to  cartilage  are 
not  distinguishable,  by  any  means  we  at  present  possess, 
in  any  important  respect  from  those  which  give  rise  to 
epidermis. 

Nevertheless,  the  similar  form  must  disguise  a  different 
molecular  machinery,  inasmuch  as  the  two,  when  developing 
under  the  conditions  of  temperature,  oxygen,  and  nutriment 
to  which  they  are  exposed  in  the  living  body,  produce  tissues 
which  differ  so  widely  as  cartilage  and  epidermis. 

The  embryonic  cartilage  cells,  like  the  embryonic  epider- 
mal cells,  are  living  organisms  in  which  certain  definitely 
limited  possibilities  of  growth  and  metamorphosis  are 
inherent,  as  they  are  in  those  equally  simple  organisms,  the 
spores  of  the  common  moulds,  Penicillium  and  Mucor. 
Given  the  proper  external  conditions,  the  latter  grow  into 
moulds  of  two  different  kinds,  while  the  former  grow  into 
cartilage  and  horny  plates. 


CONNECTIVE  TISSUES 


49 


12.    The  Structure  of  Connective  Tissues. 

(i)  Areolar  Tissue.  —  If  a  specimen  of  the  loose  subcuta- 
neous tissue  which  binds  the  skin  to  the  body,  or  of  the 
similar  tissue  from  between  the  muscles  of  a  limb,  be  exam- 
ined, it  is  found  to  be  a  soft  stringy  substance.  If  a  small 
portion  is  carefully  spread  out  in  fluid  on  a  glass  slide  and 
examined  without  the  aid  of  any  microscope,  it  is  seen  to 
consist  of  semi-transparent  whitish  bands  and  fibres,  of  very 
various  thicknesses,  interlaced  so  as  to  form  a  network,  the 


c— 


JsAm 

iftJ 

Fig.  14.  —  Connective  Tissue  Fibres. 
a.  small  bundles  of  white  fibres:  b,  larger  bundles:  c,  single  elastic  fibres. 

meshes  of  which  are  extremely  irregular.     Hence  the  oldei 
anatomists  termed  this  tissue  areolar  or  cellular. 

When  a  specimen  of  fresh  connective  tissue  is  prepared 
for  the  microscope  in  its  own  fluid,  it  is  seen  to  consist 
largely  of  strings  and  threads  varying  extremely  in  thickness, 
which  cross  one  another  in  all  directions  and  are  often 
wavy  (Fig.  14).  A  few  of  the  threads  are  seen  to  be  sharply 
defined  fibres  of  a  strongly  refracting  substance  (Fig.  15). 

E 


5° 


ELEMENTARY    PHYSIOLOGY 


When  occurring  in  mass  the  latter  appear  yellowish  in 
color.  They  are  very  elastic  and  are  unaffected  by  even 
strong  acids  or  alkalies  or  by  prolonged  boiling.  They  are 
called  elastic  fibres. 

The  majority  of  the  threads  are  pale  and  not  darkly  con- 
toured. All  the  thicker  strings  present  a  fine  longitudinal 
striation,  due  to  their  being  composed  of  extremely  fine 
fibrilke  (Fig.  16,  A).  These  pale  threads,  whether  occur- 
ring singly  or  in  bundles,  are  known  as  white  fibres.  They 
differ  from  the  elastic  fibres  in  being  smaller  and  of  a  differ- 
ent chemical  nature.     When  subjected  to  acids  or  alkalies, 


Fig.  15.  —  Elastic  Fibres  of  Connective  Tissue,  forming  a  Luose  Xetwork. 


Obtained  by   special   preparation   from   subcutaneous    tissue.     Magnified   800 
diameters. 


they  swell  up  and  acquire  a  glassy  transparency  (Fig.  16,  B). 
When  boiled  in  water,  they  dissolve  into  gelatin,  from  which 
fact  they  are  sometimes  called  collagenous  fibres. 

With  care  certain  cells  may  also  be  seen  in  fresh,  living, 
connective  tissue,  but  they  are  most  distinctly  visible  when 
the  tissue  is  treated  with  dilute  acetic  acid.  These  cells,  or 
connective  tissue  corpuscles,  as  they  are  called  (just  as 
cartilage  cells  are  called  cartilage  corpuscles),  when  seen  in 
fresh  tissue,  care  being  taken  to  prevent  the  post-mortem 
changes  which  they  readily  undergo,  are  found  to  be  flat- 


CONNECTIVE  TISSUES 


51 


tened  plates,  almost  like  epithelial  scales,  but  with  very 
irregular  contours  (Fig.  17).  They  closely  adhere  to  and 
are,  as  it  were,  bent  round  the  convex  faces  of  the  larger 
bundles  of  white  fibres. 

Thus,  connective  tissue  resembles  cartilage  in  so  far  as  it 
consists  of  cells  separated  by  a  large  quantity  of  intercellular 
substance ;  but  this  intercellular  substance  is  soft,  areolated, 
fibrous,  and,  for  the  most  part,  either  collagenous  or  elastic, 
in  contradistinction  from  that  of  cartilage,  which  is  hard, 
solid,  laminated,  and  chondrigenous. 


A.  A  small  bundle  of  connective  tissue,  showing  longitudinal  fibrillation,  and  at 
g  and  b  encircling  (annular,  spiral)  fibres      Magnified  400  diameters. 

B.  A  similar  bundle  swollen  and  rendered  transparent  by  dilute  acid.     The  en- 
circling fibres  are  seen  at  a,  a,  a. 


Besides  these  fixed  connective  tissue  corpuscles  as  they 
are  called,  white  blood  corpuscles  (p.  126),  or  lymph  cor- 
puscles, or  bodies  exceedingly  like  them,  are  found  lying 
loose  in  the  fluid  which  occupies  the  meshes  of  the  network 
of  fibres,  and  appear  to  wander  or  travel  through  the  spaces 
of  the  network  by  virtue  of  their  power  of  amoeboid  move- 
ment. Such  cells  are  spoken  of  as  wandering  or  migratory 
cells. 

(ii)  Other  Varieties  of  Connective  Tissue.  —  Such  are  the 
characters  of  that  which  may  be  regarded  as  a  typical  speci- 
men of  connective  tissue.     But  in   different  parts  of  the 


52  ELEMENTARY   PHYSIOLOGY  less. 

body  this  tissue  presents  great  differences,  alL  of  which, 
however,  are  dependent  upon  the  different  relative  extent  to 
which  the  various  elements  of  the  tissue  are  developed. 

Thus,  {a)  the  intercellular  substance  may  be  very  much 
reduced  in  amount  in  proportion  to  the  cells,  as  is  the  case 
in  the  superficial  layer  of  the  dermis  and  some  other  places, 

(b)  The  intercellular  substance  may  be  abundant,  and 
the  white  fibres  strongly  marked  and  arranged  in  close-set 
parallel  bundles,  leaving  mere  clefts  in  place  of  the  wide 
meshes  of  ordinary  connective  tissue.  This  structure  is 
seen  in  tendons  and  most  ligaments  and  is  called  fibrous 
tissue. 


V 


Pft-:. 

% 


Fig.  17.  —  Two  Connective  Tissue  Cokpuscles. 

Each  is  seen  to  consist  of  a  branched   protoplasmic   body,  containing  a   nucleus. 
Very  highly  magnified. 


(c)  The  elastic  fibres  may  predominate,  as  in  the  vocal 
cords,  and  the  strong  ligament  {ligamentum  nuchce)  at  the 
back  of  the  neck  (Fig.  109,  b),  which  is  so  highly  developed 
in  long-necked  animals,  such  as  the  horse.  Such  tissue  is 
called  elastic  tissue. 

(d)  The  white  fibrous  or  elastic  elements  may  abound, 
but  a  greater  or  less  amount  of  chondrigenous  substance 
may  be  developed  around  the  corpuscles.  These  are  the 
fibro- cartilages  which  we  have  already  described,  and  which 
present  every  transition  between  ordinary  cartilage  and 
ordinary  connective    tissue    (epiglottis,  intervertebral   liga- 


VARIETIES   OF   CONNECTIVE  TISSUE 


53 


merits) .  Where  a  tendon  is  inserted  into  a  cartilage,  as  in 
the  case  of  the  tendo  Achi/lis,  the  powerful  tendon  that 
stretches  from  the  calf-muscle  of  the  leg  into  the  bone  of 
the  heel,  the  passage  of  the  cartilage  into  the  tendon  is 
beautifully  displayed.  The  intercellular  substance  of  the 
cartilage  gradually  takes  on  the  characters  of  that  of  the 
tendon,  and  the  corpuscles  of  the  cartilage  become  connec- 
tive tissue  corpuscles. 

(<?)  The  intercellular  substance  may  largely  disappear 
and  the  interlacing  bundles  of  collagenous  fibres  may  actu- 
ally join  together  at  the  points  where  they  cross  one  another. 
In  this  way  a  spongy  network  of  branching  fibres  may  be 
formed,  called  adenoid,  retiforiti,  or  lymphoid  tissue,  whose 
meshes  are  filled  with  fluid,  as  in  the  lymphatic  glands  (Fig. 
40,  p.  115). 

(/)  Finally,  in  many  parts  of  the  body  fatty  matter  is 
found  within  the  protoplasmic  substance  of  the  connective 
tissue  corpuscles,  just  as  we  have  seen  it  to  be  formed  in 
cartilage  corpuscles.  The  fatty  deposit,  beginning  as  mi- 
nute granules  and  droplets  of  fat,  gradually  increases  in 
amount,  at  the  same  time  distending  the  body  of  the  cell, 
until  the  latter  becomes  a  spheroidal  sac  full  of  fat,  with  the 
nucleus  pushed  to  one  side.  The  conspicuous  fatty  or 
adipose  tissue,  found  in  many  parts  of  the  body,  consists 
simply  of  an  aggregation  of  vast  numbers  of  these  modified 
cells,  held  together  by  a  vascular  framework  furnished  by 
the  connective  tissue  to  which  they  belong  (Fig.  18). 

13.  The  Development  of  Connective  Tissue.  —  In  a  young 
embryo,  the  places  in  which  connective  tissue  will  make  its 
appearance  are  occupied  by  masses  of  simple  undifferen- 
tiated nucleated  cells.  By  degrees,  the  cells  become  sepa- 
rated by  a  transparent  intercellular  substance  or  matrix, 
which  eventually  takes  on  the  form  of  white  and  of  elastic 


54 


ELEMENTARY   PHYSIOLOGY 


fibres,  the  relative  proportion  and  the  disposition  of  the  two 
varying  according  to  the  kind  of  connective  tissue  which  is 
being  formed.  As  in  the  corresponding  case  of  cartilage, 
the  exact  part  played  by  the  cells  in  the  formation  of  this 


Fig.  18. — Adipose  Tissue. 

Five  fat  cells,  held  together  by  bundles  of  connective  tissue,  f.  ;«,  the  membrane 
or  envelope  of  the  fat  cell;  «,  the  nucleus,  and  /,  the  remains  of  the  protoplasm 
pushed  aside  by  the  large  oil  drop  a.     Magnified  200  diameters. 

matrix  is  still  a  matter  of  dispute.  As  the  development 
of  the  tissue  proceeds,  the  cells  multiply  by  division  and 
assume  their  characteristic  flattened  and  irregular  forms, 
applying  themselves  to  or  rather  becoming  compressed 
between  the  bundles  of  white  fibres. 


LESSON    III 

THE  VASCULAR  SYSTEM  AND  THE  CIRCULATION 

Part  I.  — The  Blood  Vascular  System  and  the 
Circulation  of  Blood 


1.  The  Capillaries.  —Almost  all  parts  of  the  body  are 
vascular ;  that  is  to  say,  they  are  traversed  by  minute  and 
very  close-set  canals,  which  open  into  one  another  so  as  to 
constitute  a  small-meshed  network,  and  confer  upon  these 
parts  a  spongy  texture.  The  canals,  or  rather  tubes,  are 
provided  with  distinct  but  very  delicate  walls,  composed  of 


d- 


Fig.  19.  —  Capillaries. 


A,  surface  view;   B,  cut  lengthwise;   C,  cut  across;  e.c,  epithelial  cells;  n,  nuclei; 
d,  the  lumen  or  bore. 

what  at  first  sight  appears  to  be  a  structureless  membrane, 
but  is  in  reality  formed  of  a  number  of  thin  epithelial  cells, 
cemented  together  at  their  edges  (Fig.  19,  A,  e.c)  ;  in  each 
of  these  cells  lies  a  small  oval  nucleus  («). 


56  ELEMENTARY   PHYSIOLOGY  less. 

These  tubes  are  the  blood-capillaries.  They  vary  in 
diameter  from  7^  to  12/u.  (-j-jVo'  t0  20^0  °f  an  mcn)  >  rney 
are  sometimes  disposed  in  loops,  sometimes  in  long,  some- 
times in  wide,  sometimes  in  narrow  meshes  ;  and  the  diam- 
eters of  these  meshes,  or,  in  other  words,  the  interspaces 
between  the  capillaries,  are  sometimes  hardly  wider  than 
the  diameter  of  a  capillary,  sometimes  many  times  as  wide. 
(See  Figs.  36,  48,  66,  and  72.)  These  interspaces  are  occu- 
pied by  the  substance  of  the  tissue  which  the  capillaries  per- 
meate, so  that  the  ultimate  anatomical  components  of  every 
part  of  the  body  are,  strictly  speaking,  outside  the  vessels, 
or  extra-vascular. 

But  there  are  certain  parts  of  the  body  in  which  these 
blood-capillaries  are  absent.  These  are  the  epidermis  and 
epithelium,  the  nails  and  hairs,  the  substance  of  the  teeth, 
and  to  a  certain  extent  the  cartilages  and  the  transparent 
coat  (cornea)  of  the  eye  in  front ;  which  may  and  do  attain 
a  very  considerable  thickness  or  length,  and  yet  contain  no 
blood-vessels.  However,  since  we  have  seen  that  all  the 
tissues  are  really  extra- vascular,  these  differ  only  in  degree 
from  the  rest.  The  circumstance  that  all  the  tissues  are 
outside  the  vessels  by  no  means  interferes  with  their  being 
bathed  by  the  fluid  which  is  inside  the  vessels.  In  fact,  the 
walls  of  the  capillaries  are  so  exceedingly  thin  that  their 
fluid  contents  readily  exude  through  the  delicate  membrane 
of  which  they  are  composed,  and  irrigate  the  tissues  in 
which  they  lie. 

2.  The  Arteries  and  Veins. — The  capillary  tubes  so  far 
described  contain,  during  life,  the  red  fluid,  blood,  and  are 
continued,  on  opposite  sides,  into  somewhat  larger  tubes, 
with  thicker  walls,  which  are  the  smallest  arteries,  on  the 
one  side,  and  veins,  on  the  other,  and  these  again  join 
on  to  larger  arteries  and  veins,  which  ultimately  communi- 


IU  STRUCTURE   OF   ARTERIES  57 

cate  by  a  few  principal  arterial  and  venous  trunks  with  the 
heart. 

The  mere  fact  that  the  walls  of  these  vessels  are  thicker 
than  those  of  the  capillaries  constitutes  an  important  differ- 
ence between  the  capillaries  and  the  small  arteries  and 
veins ;  for  the  walls  of  the  latter  are  thus  rendered  far  less 
permeable  to  fluids,  and  that  thorough  irrigation  of  the  tis- 
sues, which  is  effected  by  the  capillaries,  cannot  be  performed 
by  them. 

The  most  important  difference  between  these  vessels  and 
the  capillaries,  however,  lies  in  the  circumstance  that  their 
walls  are  not  only  thicker,  but  also  more  complex,  being 
composed  of  several  coats,  one,  at  least,  of  which  is  muscu- 
lar. The  number,  arrangement,  and  even  nature  of  these 
coats  differ  according  to  the  size  of  the  vessels,  and  are  not 
the  same  in  the  veins  as  in  the  arteries,  though  the  smallest 
veins  and  arteries  tend  to  resemble  each  other. 

(i)  The  Structure  of  an  Artery.  —  If  we  take  one  of  the 
smallest  arteries,  we  find,  first,  a  very  delicate  lining  of  cells 
constituting  a  sort  of  epithelium  continuous  with  the  celk 
which  form  the  entire  thickness  of  the  wall  of  the  capillaries 
(Figs.  20,  21).  Outside  this  comes  the  muscular  coat,  con- 
sisting of  a  thin  layer  of  muscle  fibres  of  the  kind  called  plain 
or  non- striated  (p.  310),  made  up  of  flattened  spindle-shaped 
cells  with  an  elongated  nucleus,  wrapped  round  the  vessel  at 
right  angles  to  its  length.  Outside  this  muscular  coat  is  a 
thin  layer  of  fibrous  connective  tissue,  intermixed  with  a 
variable  amount  of  fibres  of  elastic  tissue.  The  larger  arte- 
ries are  similarly  composed  of  three  layers  or  coats,  which 
are,  however,  thicker  and  more  complex  in  structure.  The 
muscular  layer  is  very  greatly  thickened,  and  elastic  tissue 
permeates  all  the  layers. 

We  thus  see  that  arteries  are  strong,  muscular,  and  elastic. 


58 


ELEMENTARY   PHYSIOLOGY 


The  largest  arteries  are,  as  a  rule,  characteristically  more 
elastic  than  the  smaller,  while  in  the  latter  the  muscular  tis- 
sue is  present  in  large  amount  relatively  to  the  elastic  tissue. 
The  significance  of  this  difference  will  become  apparent 
later  on  (see  pp.  87  and  91). 

The  plain  muscular  fibres  in  the  arterial  wall  possess  that 
same  power  of  contraction,  or  shortening  in  the  long,  and 
broadening  in  the  narrow,  directions,  which,  as  was  stated 
in  the  first  Lesson,  is  the  special 
property  of  muscular  tissue.  And 
when  they  exercise  this  power,  they, 
of  course,  narrow  the  calibre  of  the 
vessel,  just  as  squeezing  it  with  the 
hand  or  in  any  other  way  would  do ; 
and  this  contraction  may  go  so  far, 
as,  in  some  cases,  to  reduce  the  cav- 
ity of  the  vessel  almost  to  nothing, 
and  to  render  it  practically  imper- 
vious. 

The  state  of  contraction  of  these 
muscles  of  the  small  arteries  is  regu- 
lated, like  that  of  other  muscles,  by 
their  nerves ;  or,  in  other  words,  the 
nerves  supplied  to  the  vessels  deter- 
mine whether  the  passage  through 
these  tubes  shall  be  wide  and  free, 
or  narrow  and  obstructed.  Thus,  while  the  small  arteries 
lack  the  function,  which  the  capillaries  possess,  of  directly 
irrigating  the  tissues  by  transudation,  they  possess  that  of 
regulating  the  supply  of  fluid  to  the  irrigators  or  capil- 
laries themselves.  The  contraction,  or  dilation,  of  the 
arteries  which  supply  a  set  of  capillaries,  comes  to  the 
same  result  as  lowering  or  raising  the  sluice-gates  of  a  sys- 


Fig.  20.  —  Diagram    illus- 
trating   the    Structure 
of  an  Artery. 
e,  inner  coat  of  epithelium; 
m,    middle    coat    of   smooth 
muscle,  here    shown    for  the 
sake  of  simplicity  as  a  single 
layer  of  cells;   c,  outer  coat 
of  connective  tissue,  showing 
fibres  and  cells. 


in  THE   VALVES   OF  THE  VEINS  59 

tern  of  irrigation-canals.  Thus  the  one  great  and  all-impor- 
tant use  of  the  muscular  tissue  of  the  smaller  arteries  is  to 
determine  and  control  the  supply  of  blood  to  each  part  of  the 
body,  according  to  the  varying  needs  of  that  part. 

The  smaller  arteries  and  veins  severally  unite  into,  or  are 
branches  of,  larger  arterial  or  venous  trunks,  which  again 
spring  from  'or  unite  into  still  larger  ones,  and  these,  at 
length,  communicate  by  a  few  principal  arterial  and  venous 
trunks  with  the  heart. 

(ii)  The  Structure  of  a  Vein.  —  The  wall  of  a  vein  is  struc- 
turally similar  to  that  of  an  artery  in  so  far  that  it  consists 
essentially  of  the  same  three  layers  or  coats,  but  the  distinc- 
tion between  the  middle  and  outer  coats,  so  easily  made 
out  in  an  artery,  is  usually  very  obscure  in  a  vein  or  even 
altogether  wanting  in  some  veins  (Fig.  21).  It  differs  from 
that  of  an  artery  chiefly  in  the  fact  that  it  is  thinner,  less 
muscular  and  less  elastic,  and  contains  relatively  more 
connective  tissue.  Hence  the  walls  of  a  vein  collapse  or 
fall  together  when  the  vessel  is  empty,  whereas  those  of 
an  artery  do  not. 

This  is  one  great  difference  between  the  arteries  and  the 
veins ;  the  other  is  the  presence  of  what  are  termed  valves 
in  a  great  many  of  the  veins,  especially  in  those  which  lie  in 
muscular  parts  of  the  body.  They  are  absent  in  the  largest 
trunks,  such  as  the  superior  and  inferior  vena  cava,  and  in 
the  smallest  branches,  as  also  in  the  portal,  pulmonary,  and 
cerebral  veins,  and  in  those  of  the  bones. 

These  valves  (Fig.  22)  are  pouch-like  folds  of  the  inner 
wall  of  the  vein.  The  bottom  of  the  pouch  is  turned 
towards  those  capillaries  from  which  the  vein  springs.  The 
free  edge  of  the  pouch  is  directed  the  other  way,  or  towards 
the  heart.  The  action  of  these  pouches  is  to  impede  the 
passage  of  any  fluid  from  the  heart  towards  the  capillaries, 


6o 


ELEMENTARY   PHYSIOLOGY 


while  they  do  not  interfere  with  fluid  passing  in  the  oppo- 
site direction.  The  working  of  some  of  these  valves  may  be 
very  easily  demonstrated  in  the  living  body.  When  the  arm 
is  bared,  blue  veins  may  be  seen  running  from  the  hand, 
under  the  skin,  to  the  upper  arm.  The  diameter  of  these 
veins  is  pretty  even,  and  diminishes  regularly  towards  the 
hand,  so  long  as  the  current  of  the  blood,  which  is  running 
in  them,  from  the  hand  to  the  upper  arm,  is  uninterrupted. 


Fig.  21. — Transverse  Section  of  an  Artery  and  of  a  Corresponding 
Vein. 

A,  artery;  V,  vein:  e.c,  epithelial  cells;  m,  muscular  (middle)  coat;  c,  connective 
tissue  (outer)  coat;   «,  nuclei  of  epithelial  cells. 


But  if  a  finger  be  pressed  upon  the  upper  part  of  one  of 
these  veins,  and  then  passed  downwards  along  it,  so  as  to 
drive  the  blood  which  it  contains  backwards,  sundry  swell- 
ings, like  little  knots,  will  suddenly  make  their  appearance  at 
several  points  in  the  length  of  the  vein,  where  nothing  of 
the  kind  was  visible  before.  These  swellings  are  simply 
dilatations  of  the  wall  of  the  vein,  caused  by  the  pressure 
of  the  blood  on  that  wall,  above  a  valve  which  opposes  its 
backward  progress.  The  moment  the  backward  impulse 
ceases    the    blood    flows    on    again ;    the   valve,   swinging 


in       GENERAL   ARRANGEMENT   OF   BLOOD-VESSELS       61 

back  towards  the  wall  of  the  vein,  affords  no  obstacle  to 
its  progress,  and  the  distension  caused  by  its  pressure  dis- 
appears. 

These  valves  play  an  important  part  in  determining  the 
flow  of  blood  along  the  veins  from  the  capillaries  towards 
the  heart.  This  they  do,  not  in  virtue  of  any  propulsive 
power  of  their  own,  but  in  response  to  pressure  applied  to  the 


Fig.  22. —  The  Valves  of  Veins. 

C,  H,  C,  H,  diagrammatic  sections  of  veins  with  valves.  In  the  upper  figure  the 
blood  is  supposed  to  be  flowing  in  the  direction  of  the  arrow,  towards  the  heart;  in 
the  lower,  back  towards  the  capillaries;  C,  capillary  side;  H,  heart  side.  A,  a  vein 
laid  open  to  show  a  pair  of  pouch-shaped  valves. 


veins  f?'om  their  exterior.  Such  pressure  tends  to  squeeze 
the  blood  out  of  that  part  of  the  vein  on  which  it  is  brought 
"to  bear  ;  but  since  the  valves  only  open  towards  the  heart, 
the  blood  is  thereby  driven  on  in  the  desired  direction. 
Hence  it  is  that  the  valves  are  most  numerous  in  those  veins 
which  are  most  subject  to  muscular  pressure,  such  as  those 
of  the  arms  and  legs. 

The  only  arteries  which  possess  valves  are  the  primary 
trunks  —  the  aorta  and  pulmonary  artery — which  spring 
from  the  heart,  but  these  valves,  since  they  really  belong  to 
the  heart,  will  be  best  considered  with  that  organ. 

3.  The  General  Arrangement  of  Blood-vessels  in  the 
Body.  —  It  will  now  be  desirable  to  take  a  general  view  of 
the  arrangement  of  all  these  different  vessels,  and  of  their 


62 


ELEMENTARY    PHYSIOLOGY 


-IhJk 


TTCI 


Fig.  23  — Diagram  of  the  Heart  and  Vessels,  with  the  Course  of  the 
Circulation,  viewed  from  behind,  so  that  the  proper  left  of  the 
Observer  corresponds  with  the  left  side  of  the  Heart  in  the  Diagram. 

L.A.  left  auricle;  L.V.  left  ventricle;  Ao.  aorta;  A1,  arteries  to  the  upper  part 
of  the  body;  A2,  arteries  to  the  lower  part  of  the  body;  H.A.  hepatic  artery,  which 
supplies  the  liver  with  part  of  its  blood:  I'',  veins  of  the  upper  part  of  the  body; 
V'1,  veins  of  the  lower  part  of  the  body;  V.P.  portal  vein;  H.V.  hepatic  vein; 
V.C.I,  inferior  vena  cava;  V.C.S.  superior  vena  cava;  R.A.  right  auricle;  R.V. 
right  ventricle;  P. A.  pulmonary  artery;  Lg.  lung;  P.  V.  pulmonary  vein;  Let, 
lacteals;  Ly,  lymphatics;  Th.D.  thoracic  duct:  Al.  alimentary  canal:  Lr.  liver. 
The  arrows  indicate  the  course  of  the  blood,  lymph,  and  chyle.  The  vessels  which 
contain  arterial  blood  have  dark  contours,  while  those  which  carry  venous  blood  have 
light  contours. 


HI  HEART   AND   VESSELS  6j 

relations  to  the  great  central  organ  of  the  vascular  system — ■ 
the  heart  (Fig.  23). 

All  the  veins  of  every  part  of  the  body,  except  the  lungs, 
the  heart  itself,  and  certain  viscera  of  the  abdomen,  join 
together  into  larger  veins,  which,  sooner  or  later,  open  into 
one  of  two  great  trunks  (Fig.  23,  V.C.S.,  V.C.I.) ,  termed 
the  superior  and  the  inferior  vena  cava  ;  these  in  turn  open 
into  the  upper  or  broad  end  of  the  right  half  of  the  heart. 

All  the  arteries  of  every  part  of  the  body,  except  the  lungs, 
are  more  or  less  remote  branches  of  one  great  trunk  —  the 
aorta  (Fig.  23,  Ac),  which  springs  from  the  lower  division 
of  the  left  half  of  the  heart. 

The  arteries  of  the  lungs  are  branches  of  a  great  trunk, 
the  pulmonary  artery  (Fig.  23,  P.A.),  springing  from  the 
lower  division  of  the  right  side  of  the  heart.  The  veins  of 
the  lungs,  on  the  contrary,  open  by  four  trunks,  the  pulmo- 
nary veins  (Fig.  23,  P.V.),  into  the  upper  part  of  the  left 
side  of  the  heart. 

Thus,  the  venous  trunks  open  into  the  upper  division  of 
each  half  of  the  heart :  those  of  the  body  in  general  into 
that  of  the  right  half,  those  of  the  lungs  into  that  of  the  left 
half;  while  the  arterial  trunks  spring  from  the  lower  moieties 
of  each  half  of  the  heart :  that  for  the  body  in  general  from 
the  left  side,  and  that  for  the  lungs  from  the  right  side. 

Hence  it  follows  that  the  great  artery  of  the  body,  and  the 
great  veins  of  the  body,  are  connected  with  opposite  sides 
of  the  heart ;  and  the  great  artery  of  the  lungs  and  the 
great  veins  of  the  lungs  also  with  opposite  sides  of  that 
organ.  On  the  other  hand,  the  veins  of  the  body  open  into 
the  same  side  of  the  heart  as  the  artery  of  the  lungs,  and 
the  veins  of  the  lungs  open  into  the  same  side  of  the  heart 
as  the  artery  of  the  body. 

The  arteries  which  open  into  the  capillaries  of  the  sub- 


64 


ELEMENTARY    PHYSIOLOGY 


Fig.  24. — Heart  of  Sheep,  as  seen  aftek  Removal  from  the  Body,  lying 
upon  the  two  Lungs.  The  Pericardium  has  been  cut  away,  but  no  other 
dissection  made. 

R.A.  auricular  appendage  of  right  auricle;  L.A.  auricular  appendage  of  left 
auricle;  R.V.  right  ventricle;  L.V.  left  ventricle;  S.V.C.  superior  vena  cava; 
I.V.C.  inferior  vena  cava:  P. A.  pulmonary  artery;  Ao,  aorta;  A'o',  innominate 
branch  from  aorta  dividing  into  subclavian  and  carotid  arteries;  L.  lung;  TV.  trachea, 
1,  solid  cord  often  present,  the  remnant  of  a  communication,  open  in  the  embryo, 
between  the  pulmonary  artery  and  aorta.  2,  masses  of  fat  at  the  bases  of  the 
ventricle  hiding  from  view  the  greater  part  of  the  auricles.  3,  line  of  fat  marking 
the  division  between  the  two  ventricles.     4,  mass  of  fat  covering  end  of  trachea. 


in  BLOOD-VESSELS  65 

stance  of  the  heart  are  called  coronary  arteries,  and  arise, 
like  the'  other  arteries,  from  the  aorta,  but  quite  close  to  its 
origin,  just  beyond  the  semilunar  valves.  But  the  coronary 
vein,  which  is  formed  by  the  union  of  the  small  veins  which 
arise  from  the  capillaries  of  the  heart,  does  not  open  into 
either  of  the  venae  cavge,  but  pours  the  blood  which  it  con- 
tains directly  into  the  division  of  the  heart  into  which  these 
venae  cavae  open  —  that  is  to  say,  into  the  right  upper  division 
(Fig.  30,  b). 

The  abdominal  viscera  referred  to  above,  the  veins  of 
which  do  not  take  the  usual  course,  are  the  stomach,  the 
intestines,  the  spleen,  and  the  pancreas.  These  veins  all 
combine  into  a  single  trunk,  which  is  termed  the  portal  vein 
(Fig.  23,  V.P.),  but  this  trunk  does  not  open  into  the  infe- 
rior vena  cava.  On  the  contrary,  having  reached  the  liver, 
it  enters  the  substance  of  that  organ,  and  breaks  up  into 
an  immense  multitude  of  capillaries,  which  ramify  through 
the  liver,  and  become  connected  with  those  into  which  the 
artery  of  the  liver,  called  the  hepatic  artery  (Fig.  23,  H.A.), 
branches.  From  this  common  capillary  meshwork  veins 
arise,  and  unite,  at  length,  into  a  single  trunk,  the  hepatic 
vein  (Fig.  23,  H.V.),  which  emerges  from  the  liver,  and 
opens  into  the  inferior  vena  cava.  The  flow  of  blood  from 
the  abdominal  viscera  through  the  liver  to  the  hepatic  vein 
is  called  the  portal  circulation.  The  portal  vein  is  the  only 
great. vein  in  the  body  which  branches  out  and  becomes  con- 
tinuous with  the  capillaries  of  an  organ,  like  an  artery.  But 
certain  small  veins  in  the  kidney  are  similarly  arranged 
(p.  205). 

The  shortest  possible  course  which  any  particle  of  the 
blood  can  take  in  order  to  pass  from  one  side  of  the  heart 
to  the  other,  is  to  leave  the  aorta  by  one  of  the  coronary 
arteries,  and  return    to   the   right   auricle   by  the   coronary 

F 


66 


ELEMENTARY   PHYSIOLOGY 


vein.  And  in  order  to  pass  through  the  greatest  possible 
number  of  capillaries  and  return  to  the  point  from  which 
it  started,  a  particle  of  blood  must  leave  the  heart  by  the 
aorta  and  traverse  the  arteries  which  supply  the  alimentary 
canal,  spleen,  and  pancreas.  It  then  enters,  first,  the  capil- 
laries of  these  organs;  secondly,  the  capillaries  of  the  liver; 
and,  thirdly,  after  passing  through  the  right  side  of  the  heart, 
the  capillaries  of  the  lungs,  from  which  it  returns  to  the  left 
side  and  eventually  to  the  aorta. 


Fig. 


25- 


Transverse  Section   of   the  Chest,    with    Heart  and    Lungs    in 
Place.     (A  little  diagrammatic.) 

D.  V.  dorsal  vertebra,  or  joint  of  the  backbone;  Ao,  Ao' ,  aorta,  the  top  of  its  arch 
being  cut  away  in  this  section;  S.C.  superior  vena  cava;  P. A.  pulmonary  artery, 
divided  into  a  branch  for  each  lung;  L.P.,  R.P.  left  and  right  pulmonary  veins;  Br, 
bronchi;  R.L.,  L.L.  right  and  left  lungs;  CE,  the  gullet  or  oesophagus;  p,  outer  bag 
of  pericardium;  //,  the  two  layers  of  pleura;  v,  azygos  vein. 

4.  The  Heart. — The  heart  (Figs.  24  and  26),  to  which 
all  the  vessels  in  the  body  have  now  been  directly  or  indi- 
rectly traced,  is  an  organ,  the  size  of  which  is  usually  roughly 
estimated  as  equal  to  that  of  the  closed  fist  of  the  person  to 
whom  it  belongs,  and  which  has  a  broad  end  turned  upwards 
and  backwards,  and  rather  to  the  right  side,  called  its  base  ; 


in  THE    HEART  6? 

and  a  pointed  end  which  is  called  its  apex,  turned  down- 
wards and  forwards,  and  to  the  left  side,  so  as  to  lie  oppo- 
site the  interval  between  the  fifth  and  sixth  ribs. 

It  is  lodged  between  the  lungs,  nearer  the  front  than  the 
back  wall  of  the  chest,  and  is  inclosed  in  a  sort  of  double 
bag,  the  pericardium  (Fig.  25,  /).  One-half  of  the 
double  bag  is  closely  adherent  to  the  heart  itself,  forming  a 
thin  coat  upon  its  outer  surface.  At  the  base  of  the  heart, 
this  half  of  the  bag  passes  on  to  the  great  vessels  which 
spring  from,  or  open  into,  that  organ  ;  and  becomes  con- 
tinuous with  the  other  half,  which  loosely  envelopes  both 
the  heart  and  the  adherent  half  of  the  bag.  Between  the 
two  layers  of  the  pericardium,  consequently,  there  is  a  com- 
pletely closed,  narrow  cavity,  lined  by  an  epithelium,  and 
containing  in  its  interior  a  small  quantity  of  clear  fluid,  the 
pericardial  fluid.1 

The  outer  layer  of  the  pericardium  is  firmly  connected 
below  with  the  upper  surface  of  the  diaphragm. 

But  the  heart  cannot  be  said  to  depend  greatly  upon  the 
diaphragm  for  support,  inasmuch  as  the  great  vessels  which 
issue  from  or  enter  it  —  and  for  the  most  part  pass  upwards 
from  its  base  —  help  to  suspend  and  keep  it  in  place. 

Thus  the  heart  is  coated,  outside,  by  one  layer  of  the 
pericardium.  Inside,  it  contains  two  great  cavities  or 
"  divisions,"  as  they  have  been  termed  above,  a  right  and 
a  left  cavity,  completely  separated  by  a  fixed  partition,  which 
extends  from  the  base  to  the  apex  of  the  heart ;  and  con- 
sequently, having  no  direct  communication  with  one  another. 

1  This  fluid,  like  that  contained  in  the  peritoneum,  pleura,  and  other 
shut  sacs  of  a  similar  character  to  the  pericardium,  used  to  be  called  serum  ; 
whence  the  membranes  forming  the  walls  of  these  sacs  are  frequently  termed 
serous  membranes.  The  fluid  is,  however,  in  reality  a  form  of  lymph.  (See 
p.  142.) 


68 


ELEMENTARY   PHYSIOLOGY 


Each  of  these  two  great  cavities  is  further  subdivided,  not 
longitudinally  but  transversely,  by  a  movable  partition.  The 
cavity  above  the  transverse  partition  on  each  side  is  called 
the  auricle ;  the  cavity  below,  the  ventricle  —  right  or  left 
as  the  case  may  be. 

Each  of  the  four  cavities  has  the  same  capacity,  and  is 
capable  of  containing  from  four  to  six  cubic  inches  of  water 
(70  to  100  cubic  centimetres).  The  walls  of  the  auricles 
are  much  thinner  than  those  of  the  ventricles.     The  wall 


RJlVi  n£  T    Sil^*^    J*& 


Fig.  26. — The  Heakt,  Great  Vessels,  and  Lungs.     (Front  View.) 

R.V.  right  ventricle;  L.V.  left  ventricle;  R.A.  right  auricle;  L.A.  left  auricle; 
Ao,  aorta;  P. A.  pulmonary  artery;  P.  V.  pulmonary  veins;  R.L.  right  lung;  L.L. 
left  lung;  V.S.  vena  cava  superior;  S.C.  subclavian  vessels;  C.  carotid  arteries; 
R.J.V.  and  L.J.V.  right  and  left  jugular  veins;  V.I,  vena  cava  inferior;  T.  trachea; 
B.  bronchi. 

All  the  great  vessels  but  those  of  the  lungs  are  cut. 

of  the  left  ventricle  is  much  thicker  than  that  of  the  right 
ventricle  ;  but  no  such  difference  is  perceptible  between  the 
two  auricles  (Figs.  27  and  28,  1  and  3). 

In  fact,  as  we  shall  see,  the  ventricles  have  more  work 
to  do  than  the  auricles,  and  the  left  ventricle  more  to  do 


in  THE   VALVES   OF  THE   HEART  69 

than  the  right.  Hence  the  ventricles  have  more  muscular 
substance  than  the  auricles,  and  the  left  ventricle  more  than 
the  right ;  and  it  is  this  excess  of  muscular  substance'  which 
gives  rise  to  the  excess  of  thickness  observed  in  the  left 
ventricle. 

At  the  junction  between  the  auricles  and  ventricles,  the 
apertures  of  communication  between  their  cavities,  called  the 
auriculo-ventricular  apertures,  are  strengthened  by  fibrous 
rings  of  connective  tissue.  To  these  rings  the  movable  par- 
titions, or  valves,  between  the  auricles  and  ventricles,  the 
arrangement  of  which  must  next  be  considered,  are  attached. 

5.  The  Valves  of  the  Heart. — There  are  three  of  these 
partitions  attached  to  the  circumference  of  the  right  auric- 
ulo-ventricular aperture,  and  two  to  that  of  the  left  (Figs. 
27,  28,  29,  30,  tv,  mv).  Each  is  a  broad,  thin,  but  very 
tough  and  strong  triangular  fold  of  connective  tissue,  at- 
tached by  its  base,  which  joins  on  to  its  fellow,  to  the 
auriculo-ventricular  fibrous  ring,  and  hanging  with  its  point 
downwards  into  the  ventricular  cavity.  On  the  right  side 
there  are,  therefore,  three  of  these  broad,  pointed  mem- 
branes, whence  the  whole  apparatus  is  called  the  tricuspid 
valve.  On  the  left  side  there  are  but  two,  which,  when 
detached  from  all  their  connections  but  the  auriculo-ven- 
tricular ring,  look  something  like  a  bishop's  mitre,  and 
hence  bear  the  name  of  the  mitral  valve. 

The  edges  and  apices  of  the  valves  are  not  completely 
free  and  loose.  On  the  contrary,  a  number  of  fine,  but 
strong,  tendinous  cords,  called  chordae  tendiiieae,  connect 
them  with  some  column-like  elevations  of  the  fleshy  sub- 
stance of  the  walls  of  the  ventricle,  which  are  termed  papil- 
lary muscles  (Figs.  27  and  28,  pp)  ;  similar  column-like 
elevations  of  the  walls  of  the  ventricles,  with  no  chordse 
tendinese  attached  to  them,  are  called  columnae  carneae. 


7o 


ELEMENTARY   PHYSIOLOGY 


It  follows,  from  this  arrangement,  that  the  valves  oppose 
no  obstacle  to  the    passage   of  fluid    from  the  auricles  to 


Fig  27.  —  Right  Side  of  the  Heart  of  a  Sheep  (laid  open). 

R.A.  cavity  of  right  auricle;  S.V.C.  superior  vena  cava;  I  V.C.  inferior  vena 
cava  (a  style  has  been  passed  through  each  of  these) ;  «,  a  style  passed  from  the 
auricle  to  the  ventricle  through  the  auriculo-ventricular  orifice;  b,  a  style  passed  into 
the  coronary  vein 

R.V.  cavity  of  the  right  ventricle;  tv,  tv,  two  flaps  of  the  tricuspid  valve;  the 
third  is  dimly  seen  behind  them,  the  style  a  passing  between  the  three.  Between  the 
two  flaps,  and  attached  to  them  by  chorda  tendinece,  is  seen  a  papillary  muscle,//, 
cut  away  from  its  attachment  to  that  portion  of  the  wall  of  the  ventricle  which  has 
been  removed.  Above,  the  ventricle  terminates  somewhat  like  a  funnel  in  the  pul- 
monary artery,  P  A.  One  of  the  pockets  in  the  semilunar  valve,  sv,  is  seen  in  its 
entirety,  another  partially. 

1,  the  wall  of  the  ventricle  cut  across;  2,  the  position  of  the  auriculo-ventricular 
ring;  3  the  wall  of  the  auricle;  4,  masses  of  fat  lodged  between  the  auricle  and  pul- 
monary artery. 

the  ventricles  ;  but  if  any  should  be  forced  the  other  way, 
it  will  at  once  get  between  the  valve  and  the  wall  of  the 


THE    VALVES    OF   THE    HEART 


7i 


heart,  and  drive  the  valve  backwards  and  upwards.  Partly 
because  they  soon  meet  in  the  middle  and  oppose  one 
another's  action,  and    partly  because  the  chordce  tendinece 


Ac 

Ao 


ijJN 


Fig.  28.  —  Left  Side  of  the  Heart  of  a  Sheep  (laid  open). 

P.  V.  pulmonary  veins  opening  into  the  left  auricle  by  four  openings,  as  shown  by 
the  styles',  a,  a  style  passed  from  auricle  into  ventricle  through  the  auriculo-ventricu- 
lar  orifice;  6,  a  style  passed  into  the  coronary  vein,  which,  though  it  has  no  connec- 
tion with  the  left  auricle,  is,  from  its  position,  necessarily  cut  across  in  thus  laying 
open  the  auricle. 

mv,  the  two  flaps  of  the  mitral  valve  (drawn  somewhat  diagrammatically) ;  //, 
papillary  muscles,  belonging  as  before  to  the  part  of  the  ventricle  cut  away;  c,  a  style 
passed  from  ventricle  into  Ao,  aorta;  Ao1,  branch  of  aorta  (see  Fig.  24,  A  V) ;  P. A. 
pulmonary  artery;  S.l'.C.  superior  vena  cava. 

i,  wall  of  ventricle  cut  across;  2,  wall  of  auricle  cut  away  around  auriculo-ventricu- 
lar  orifice;  3,  other  portions  of  auricular  wall  cut  across;  4,  mass  of  fat  around  base 
of  ventricle  (see  Fig.  24,  2). 


72 


ELEMENTARY   PHYSIOLOGY 


hold  their  edges  and  prevent  them  from  going  back  too 
far,  the  valves,  thus  forced  back,  give  rise  to  the  formation 
of  a  complete  transverse  partition  between  the  ventricle 
and  the  auricle,  through  which  no  fluid  can  pass. 

Ao 


RAV 


Fig.  29. —  View  of  the  Orifices  of  the  Heart  from  below,  the  Whole  of 
the  Ventricles  having  been  cut  away. 

R.A.V.  right  auriculo-ventricular  orifice  surrounded  by  the  three  flaps,  t.v.  1, 
t.v.  2,  t.v.  3,  of  the  tricuspid  valve;  these  are  stretched  by  cords  attached  to  the 
chorda  te?tdinece. 

L  A.V.  left  auriculo-ventricular  orifice  surrounded  in  the  same  way  by  the  two 
flaps,  m.v.  1,  m.v.  2,  of  mitral  valve;  P.  A.  the  orifice  of  the  pulmonary  artery,  the 
semilunar  valves  having  met  and  closed  together;  Ao,  the  orifice  of  the  aorta  with 
its  semilunar  valves.  The  shaded  portion,  leading  from  R.A.V.  to  P. A.,  represents 
the  funnel  seen  in  Fig.  27. 

Where  the  aorta  opens  into  the  left  ventricle,  and  where 
the  pulmonary  artery  opens  into  the  right  ventricle,  another 
valvular  apparatus  is  placed,  consisting  in  each  case  of 
three  pouch-like  valves  called  the  semilunar  valves  (Fig. 
27,  s.v. ;  Figs.  29  and  30,  Ao,  P. A.),  which  are  similar  to 
those  of  the  veins.     Since  they  are   placed   on   the  same 


Ill 


THE   VALVES   OF  THE   HEART 


73 


level  and  meet  in  the  middle  line,  they  completely  stop 
the  passage  when  any  fluid  is  forced  along  the  artery 
towards  the  heart.  On  the  other  hand,  these  valves  flap 
back  and  allow  any  fluid  to  pass  from  the  heart  into  the 
artery,  with  the  utmost  readiness. 

The   action   of  the   auriculo-ventricular   valves    may   be 


tvz 


Fig.  30.  —  The  Orifices  of  the  Heart  seen  from  above,  the  Auricles  and 
Great  Vessels  being  cut  away. 

P. A.  pulmonary  artery,  with  its  semilunar  valves;  Ao,  aorta,  ditto. 

R.A.V.  right  auriculo-ventricular  orifice  with  the  three  flaps  ((.v.  1,  2,  3)  of  tri- 
cuspid valve. 

L.A.V.  left  auriculo-ventricular  orifice,  with  m.v.  1  and  2,  flaps  of  mitral  valve: 
b,  style  passed  into  coronary  vein.  On  the  left  part  of  L.A.V.  the  section  of  the 
auricle  is  carried  through  the  auricular  appendage:  hence  the  toothed  appearance 
due  to  the  portions  in  relief  being  cut  across. 


demonstrated  with  great  ease  on  a  sheep's  heart,  in  which 
the  aorta  and  pulmonary  artery  have  been  tied  and  the 
greater  part  of  the  auricles  cut  away,  by  pouring  water 
into  the  ventricles  through  the  auriculo-ventricular  aperture. 
The  tricuspid  and  mitral  valves  then  usually  become  closed 


74 


ELEMENTARY  PHYSIOLOGY 


by  the  upward  pressure  of  the  water  which  gets  behind 
them.  Or,  if  the  ventricles  be  nearly  filled,  the  valves  may 
be  made  to  come  together  at  once  by  gently  squeezing  the 
ventricles.  In  like  manner,  if  the  base  of  the  aorta,  or 
pulmonary  artery,  be  cut  out  of  the  heart,  so  as  not  to 
injure  the  semilunar  valves,  water  poured  into  the  upper 
ends  of  the  vessel  will  cause  its  valves  to  close  tightly,  and 
allow  nothing  to  flow  out  after  the  first  moment. 

Thus,  the  arrangement  of  the  auriculo-ventricular  valves 
is  such,  that  any  fluid  contained  in  the  chambers  of  the 
heart  can  be  made  to  pass  through  the  auriculo-ventricular 
apertures  in  one  direction  only  :  that  is  to  say,  from  the 
auricles  to  the  ventricles.  On  the  other  hand,  the  arrange- 
ment of  the  semilunar  valves  is  such 
that  the  fluid  contents  of  the  ventri- 
cles pass  easily  into  the  aorta  and 
pulmonary  artery,  while  none  can  be 
made  to  travel  the  other  way  from 
the  arterial  trunks  to  the  ventricles. 

6.  The  Structure  of  the  Heart.  — 
The  heart  is  a  muscular  organ,  and 
the  substance  of  its  walls  is  mainly 
muscular  tissue.  Like  all  other  mus- 
cles this  tissue  is  composed  of  cells, 
and  these  cells  resemble  those  of 
non-striated  muscle,  as  it  occurs,  for 
example,  in  the  arteries  and  veins,  in 
containing  each  a  single  nucleus,  and 
possessing  no  cell-wall,  or  sarcolemma 
(Fig.  31).  But  the  cells  are  generally 
short  and  broad,  frequently  branched 
or  irregular  in  shape,  and  their  substance  is  more  or  less  dis- 
tinctly striated,  like  the  substance  of  a  striated  muscular  fibre 


Fig.  31. — Cardiac  Muscle 
Cells. 
Two  cells  isolated  from 
the  heart.  «,  nucleus;  /, 
line  of  junction  between  the 
two  cells;  p,  process  joining 
a  similar  process  of  another 
cell.  (Magnified  400  diame- 
ters.) 


Hi  THE    BEAT   OF  THE    HEART  75 

(p.  311).  Cardiac  muscle  is  hence  intermediate  in  charac- 
ter between  non-striated  and  striated  muscle,  representing 
a  higher  stage  of  differentiation  from  the  primitive  cells  than 
the  muscle  of  the  arteries  and  veins,  but  not  so  high  a  stage 
as  the  muscles  of  the  limbs.  The  cells  are  joined  by  inter- 
cellular cement  substance  into  sets  of  anastomosing  fibres, 
which  are  built  up  in  a  complex  interwoven  manner  into  the 
walls  of  the  ventricles  and  auricles. 

The  cavities  of  the  heart  are  lined,  and  the  valves  are 
covered,  by  a  smooth,  shiny  membrane  called  the  endocar- 
dium, which  consists  of  a  layer  of  connective  tissue  covered 
with  thin  flattened  cells  continuous  with  and  similar  to  those 
which  form  the  wall  of  the  capillaries  and  which  line  the 
arteries  and  veins. 

7.  The  Beat  of  the  Heart.  —  Like  all  other  muscular 
tissues,  the  substance  of  the  heart  is  contractile  ;  but,  unlike 
most  muscles,  the  heart  contains  within  itself  something 
which  causes  its  different  parts  to  contract  in  a  definite 
succession  and  at  regular  intervals. 

If  the  heart  of  a  living  animal  be  removed  from  the  body, 
it  will,  though  in  most  cases  for  a  very  short  time  only, 
unless  the  animal  be  "  cold-blooded  "  like  a  frog,  go  on 
beating  much  as  it  did  while  in  the  body.  And  careful 
attention  to  these  beats  will  show  that  they  consist  of :  — 

(1)  A  simultaneous  contraction  of  the  walls  of  both  auricles. 

(2)  Immediately  following  this,  a  simultaneous  contraction 
of  the  walls  of  both  ventricles.  (3)  Then  comes  a  pause, 
or  state  of  rest,  after  which  the  auricles  and  ventricles  con- 
tract again  in  the  same  order  as  before,  and  their  contrac- 
tions are  followed  by  the  same  pause  as  before. 

The  state  of  contraction  of  the  ventricle  or  auricle  is 
called  its  systole ;  the  state  of  relaxation,  during  which  it 
undergoes  dilation,  its  diastole. 


j6  ELEMENTARY   PHYSIOLOGY  less. 

If  the  auricular  contraction  be  represented  by  A",  the 
ventricular  by  V,  and  the  pauses  by  — ,  the  series  of  actions 
will  be  as  follows:  A~V"  — ;  AVV"  —  ;  A"VV  —  ;  etc. 
Thus,  the  contraction  of  the  heart  is  rhythmical,  two  short 
contractions  of  its  upper  and  lower  halves  respectively  being 
followed  by  a  pause  of  the  whole,  which  occupies  nearly  as 
much  time  as  the  two  contractions. 

The  period  occupied  by  one  complete  beat  and  the  pause 
is  usually  spoken  of  as  a  "cardiac  cycle."  This  cycle  is 
repeated,  or  as  we  more  ordinarily  say,  "  the  heart  beats  " 
in  an  average  healthy  adult  person  about  72  times  in  a  min- 
ute. From  this  it  follows  that  the  ordinary  duration  of  each 
beat  is  T8^  of  a  second.  Of  this  period  the  contraction  of 
the  auricles  occupies  y^  and  that  of  the  ventricles  T3T,  the 
remaining  fL  being  taken  up  by  the  pause  of  the  heart  as  a 
whole.  During  each  cycle  or  beat  the  heart  undergoes  cer- 
tain changes  of  shape  and  position,  as  to  the  details  of  which 
there  is  some  uncertainty,  but  which  are,  on  the  whole,  as  fol- 
lows. During  each  systole  the  width  of  the  heart  from  side 
to  side  and  probably  also  the  depth  from  back  to  front 
becomes  less.  The  result  of  this  is  that,  whereas  during 
diastole  the  shape  of  a  section  of  the  base  of  the  ventricles 
is  elliptical,  during  systole  it  becomes  much  more  nearly 
circular. 

The  length  of  the  heart  is  perhaps  lessened,  but  very 
slightly,  if  at  all,  during  systole,  and  the  heart  as  a  whole  is 
twisted  to  a  certain  extent  on  its  long  axis,  from  the  left  and 
behind  towards  the  front  and  right.  The  apex  is  at  the 
same  time  tilted  slightly  forward  and  is  hence  pressed  rather 
more  firmly  against  the  wall  of  the  thorax,  a  fact  of  some 
importance  in  connection  with  what  we  shall  describe  pres- 
ently as  the  "  cardiac  impulse  "  (see  p.  81). 

8.    The  Action  of  the  Valves.  —  Having  now  acquired  a 


Ill  THE   ACTION   OF  THE  VALVES  77 

notion  of  the  arrangement  of  the  different  pipes  and  reser- 
voirs of  the  circulatory  system,  of  the  position  of  the  valves, 
and  of  the  rhythmical  contractions  of  the  heart,  it  will  be 
easy  to  comprehend  what  must  happen  if,  when  the  whole 
apparatus  is  full  of  blood,  the  first  step  in  the  pulsation  of 
the  heart  occurs  and  the  auricles  contract. 

By  this  action  each  auricle  tends  to  squeeze  the  fluid 
which  it  contains  out  of  itself  in  two  directions,  —  the  one 
towards  the  great  veins,  the  other  towards  the  ventricles  ; 
and  the  direction  which  the  blood,  as  a  whole,  will  take, 
will  depend  upon  the  relative  resistance  offered  to  it  in 
these  two  directions.  Towards  the  great  veins  it  is  resisted 
by  the  mass  of  the  blood  contained  in  the  veins.  Towards 
the  ventricles,  on  the  contrary,  there  is  no  resistance  worth 
mentioning,  inasmuch  as  the  valves  are  open,  the  walls  of 
the  ventricles,  in  their  uncontracted  state,  are  flaccid  and 
easily  distended,  and  the  entire  pressure  of  the  arterial  blood 
is  taken  off  by  the  semilunar  valves,  which  are  necessarily 
closed.  The  return  of  blood  into  the  veins  is  further  checked 
by  a  contraction  of  the  great  veins  at  their  point  of  junction 
with  the  heart,  which  immediately  precedes  the  systole  of 
the  auricles,  and  is  practically  continuous  with  it. 

Therefore,  when  the  auricles  contract,  little  or  none  of  the 
fluid  which  they  contain  will  flow  back  into  the  veins  ;  all 
the  contents,  or  nearly  all,  will  pass  into  and  distend  the 
ventricles.  As  the  ventricles  fill  and  begin  to  resist  further 
distension,  the  blood,  getting  behind  the  auriculo-ventricu- 
lar  valves,  will  push  them  towards  one  another,  and  indeed 
almost  shut  them.  The  auricles  now  cease  to  contract,  and, 
immediately  that  their  walls  relax,  fresh  blood  flows  from  the 
great  veins  and  slowly  distends  them  again. 

But  the  moment  the  auricular  systole  is  over,  the  ventric- 
ular systole  begins.     The  walls  of  each  ventricle  contract 


78 


ELEMENTARY   PHYSIOLOGY 


vigorously,  and  the  first  effect  of  that  contraction  is  to  com- 
plete the  closure  of  the  auriculo-ventricular  valves  and  so  to 
stop  all  egress  towards  the  auricle  (Fig.  32).  The  pressure 
upon  the  valves  becomes  very  considerable,  and  they  might 
even  be  driven  upwards,  if  it  were  not  for  the  chordce  ten- 
dinece  which  hold  down  their  edges. 


Fig.  32.  —  Diagram  to  illustrate  the  Action  of  the  Heart. 

aur.  auricle;  vent,  ventricle;  v,  v,  veins;  a,  aorta;  m,  mitral  valve;  s,  semi- 
lunar valve. 

In  A  the  auricle  is  contracting,  ventricle  dilated,  mitral  valve  open,  semilunar 
valves  closed.  In  B  the  auricle  is  dilated,  ventricle  contracting,  mitral  valve  closed, 
semilunar  valves  open. 


As  the  contraction  continues  and  the  capacities  of  the 
ventricle  become  diminished,  the  points  of  the  wall  of  the 
heart  to  which  the  chordce  tendineaz  are  attached  approach 
the  edges  of  the  valves ;  and  thus  there  is  a  tendency  to 
allow  of  a  slackening  of  these  cords,  which,  if  it  really  took 
place,  might  permit  the  edges  of  the  valves  to  flap  back  and 
so  destroy  their  utility.  This  tendency,  however,  is  counter- 
acted by  the  chordce  tendinecu  being  connected,  not  directly 
to  the  walls  of  the  heart,  but  to  those  muscular  pillars,  the 
papillary  muscles,  which  stand  out  from  its  substance.    These 


Ill  THE   ACTION    OF  THE   VALVES  79 

muscular  pillars  shorten  at  the  same  time  as  the  substance 
of  the  heart  contracts;  and  thus,  just  so  far  as  the  contrac- 
tion of  the  walls  of  the  ventricles  brings  the  papillary  muscles 
nearer  the  valves,  do  they,  by  their  own  contraction,  pull 
the  chorda  tendincce  as  tight  as  before. 

By  the  means  which  have  now  been  described,  the  fluid 
in  the  ventricle  is  debarred  from  passing  back  into  the  auri- 
cle ;  the  whole  force  of  the  contraction  of  the  ventricular 
walls,  therefore,  is  expended  in  overcoming  the  resistance 
presented  by  the  semilunar  valves  (Fig.  32).  This  resist- 
ance is  partly  the  result  of  the  mere  weight  of  the  vertical 
column  of  blood  which  the  valves  support ;  but  is  chiefly 
due  to  the  reaction  of  the  distended  elastic  walls  of  the 
great  arteries,  for,  as  we  shall  see,  these  arteries  are  already 
so  full  that  the  blood  within  them  is  pressing  on  their  walls 
with  great  force. 

It  now  becomes  obvious  why  the  ventricles  have  so  much 
more  to  do  than  the  auricles,  and  why  valves  are  needed 
between  the  auricles  and  ventricles,  while  none  are  wanted 
between  the  auricles  and  the  veins'. 

All  that  the  auricles  have  to  do  is  to  fill  the  ventricles, 
which  offer  no  active  resistance  to  that  process.  Hence  the 
thinness  of  the  walls  of  the  auricles,  and  hence  the  need- 
lessness  of  any  auriculo-venous  valve,  the  resistance  on  the 
side  of  the  ventricle  being  so  insignificant  that  it  gives  way, 
at  once,  before  the  pressure  of  the  blood  in  the  veins. 

On  the  other  hand,  the  ventricles  have  to  overcome  a 
great  resistance  in  order  to  force  fluid  into  elastic  tubes 
which  are  already  full ;  and  if  there  were  no  auriculo-ven- 
tricular  valves,  the  fluid  in  the  ventricles  would  meet  with 
less  obstacle  in  pushing  its  way  backward  into  the  auricles 
and  thence  into  the  veins,  than  in  separating  the  semilunar 
valves.     Hence  the  necessity,  first,  of  the  auriculo-ventricu- 


So  ELEMENTARY   PHYSIOLOGY  less, 

lar  valves ;  and,  secondly,  of  the  thickness  and  strength  ot 
the  walls  of  the  ventricles.  And  since  the  aorta,  systemic 
arteries,  capillaries,  and  veins  form  a  system  of  tubes,  which, 
from  a  variety  of  causes,  offer  more  resistance  than  do  the 
pulmonary  arteries,  capillaries,  and  veins,  it  follows  that  the 
left  ventricle  needs  a  thicker  muscular  wall  than  the  right. 

Thus,  at  every  systole  of  the  auricles,  the  ventricles  are 
filled  and  the  auricles  emptied,  the  latter  being  slowly  re- 
filled by  the  pressure  of  the  fluid  in  the  great  veins,  which 
is  amply  sufficient  to  overcome  the  passive  resistance  of  the 
relaxed  auricular  walls.  And,  at  every  systole  of  the  ventri- 
cles, the  arterial  systems  of  the  body  and  lungs  receive  the 
contents  of  these  ventricles,  and  the  emptied  ventricles 
remain  ready  to  be  filled  by  the  auricles. 

9.  The  Working  of  the  Arteries.  —  We  must  now  con- 
sider what  happens  in  the  arteries  when  the  contents  of  the 
ventricles  are  suddenly  forced  into'  these  tubes  (which,  it 
must  be  recollected,  are  already  full). 

If  the  vessels  were  tubes  of  a  rigid  material,  like  gas- 
pipes,  the  forcible  discharge  of  the  contents  of  the  left  ven- 
tricle into  the  beginning  of  the  aorta  would  send  a  shock, 
travelling  with  great  rapidity,  right  along  the  whole  system 
of  tubes,  through  the  arteries  into  the  capillaries,  through 
the  capillaries  into  the  veins,  and  through  these  into  the 
right  auricle  ;  and  just  as  much  blood  would  be  driven  from 
the  end  of  the  veins  into  the  right  auricle  as  had  escaped 
from  the  left  ventricle  into  the  beginning  of  the  aorta ;  and 
that,  at  almost  the  same  instant  of  time.  And  the  same 
would  take  place  in  the  pulmonary  vessels  between  the 
right  ventricle  and  left  auricle. 

However,  the  vessels  are  not  rigid,  but,  on  the  contrary, 
very  yielding  tubes ;  and  the  great  arteries,  as  we  have 
seen,  have  especially  elastic  walls.     On  the  other  hand,  the 


in  THE   CARDIAC   IMPULSE  Si 

friction  in  the  small  arteries  and  capillaries,  which  opposes 
a  resistance  to  the  flow  of  blood,  and  is  hence  spoken  of  as 
the  peripheral  resistance,  is  so  great  that  the  blood  cannot 
pass  through  them  into  the  veins  as  quickly  as  it  escapes 
from  the  ventricle  into  the  aorta.  Hence  the  contents  of 
the  ventricle,  driven  by  the  force  of  the  systole  past  the 
semilunar  valves,  are  at  first  lodged  in  the  first  part  of  the 
aorta,  the  walls  of  which  are  stretched  and  distended  by 
the  extra  quantity  of  blood  thus  driven  into  it.  But,  as  soon 
as  the  ventricle  has  emptied  itself  and  no  more  blood  is 
driven  out  of  it  to  stretch  the  aorta,  the  elastic  walls  of  this 
vessel  come  into  play ;  they  strive  to  go  back  again  and 
make  the  tube  as  narrow  as  it  was  before ;  thus  they  return 
back  to  the  blood  the  pressure  which  they  received  from 
the  ventricle.  The  effect  of  this  elastic  recoil  of  the  arterial 
walls  is,  on  the  one  hand,  to  close  the  semilunar  valves,  and 
so  prevent  the  return  of  blood  to  the  heart,  and,  on  the 
other  hand,  to  distend  the  next  portion  of  the  aorta,  driving 
an  extra  quantity  of  blood  into  it.  And  this  second  por- 
tion, in  a  similar  way,  distends  the  next,  and  this  again  the 
next,  and  so  on,  right  through  the  whole  arterial  system. 
Thus  the  impulse  given  by  the  ventricle  travels  like  a  wave 
along  the  arteries,  distending  them  as  it  goes,  and  ulti- 
mately forcing  the  blood  through  the  capillaries  into  the 
veins,  and  so  on  to  the  heart  again. 

Several  of  the  practical  results  of  the  working  of  the  heart 
and  arteries  just  described  now  become  intelligible. 

10.  The  Cardiac  Impulse. — If  a  finger  be  placed  on 
the  chest  over  the  space  between  the  fifth  and  sixth  ribs 
on  the  left  side,  about  one  inch  below  the  left  nipple,  and 
slightly  towards  the  sternum,  a  certain  throbbing  movement 
is  perceptible,  which  is  known  as  the  "  cardiac  impulse." 
It  is  the  result  of  the  heart-beat  making  itself  felt  through 

G 


82  ELEMENTARY   PHYSIOLOGY  less. 

the  wall  of  the  chest  at  this  point,  at  the  moment  of  the 
systole  of  the  ventricles.  Even  when  the  heart  is  at  rest 
the  apex,  in  a  standing  position,  lies  close  under  and  in 
contact  with  this  part  of  the  chest- wall.  When  the  systole 
takes  place  the  muscular  substance  of  the  ventricles  be- 
comes suddenly  hard  and  tense,  as  do  all  muscles  when 
they  contract.  At  tne  same  time  the  apex  of  the  heart,  as 
the  result  of  the  peculiar  movements  already  described 
(p.  76),  is  brought  into  still  firmer  contact  with  the  chest- 
wall.  The  cardiac  impulse  is  the  outcome  of  this  sudden 
hardening  of  the  ventricular  walls,  aided  by  their  closer  con- 
tact with  the  wall  of  the  chest  at  the  moment  when  the  hard  • 
ening  takes  place.  It  is  ?wt  due,  as  is  so  frequently  stated, 
to  the  heart  striking  or  tapping  against  the  chest-wall. 

11.  The  Sounds  of  the  Heart.  —  If  the  ear  be  applied 
over  the  heart,  certain  sounds  are  heard,  which  recur  with 
great  regularity,  at  intervals  corresponding  with  those  be- 
tween every  two  beats.  First  comes  a  longish  dull  booming 
sound ;  then  a  short  sharp  sound,  then  a  pause,  then  the 
long,  then  the  sharp  sound,  then  another  pause  :  and  so  on. 
These  sounds  are  usually  likened  to  the  pronunciation  of 
the  syllables  "  lubb,  dup."  There  are  many  different  opin- 
ions as  to  the  cause  of  the  first  sound ;  some  physiologists 
regard  it  as  a  muscular  sound  caused  by  the  contraction  of 
the  muscular  fibres  of  the  ventricle,  while  others  believe  it 
to  be  due  to  the  vibration  of  the  auriculo-ventricular  valves, 
when  they  become  suddenly  tense  or  stretched  as  the 
ventricles  begin  to  contract.  In  reality  the  first  sound  has 
probably  a  double  origin,  being  partly  muscular  and  partly 
valvular,  and  this  view  is  borne  out  by  the  following  facts. 
The  sound  is  given  out  during  the  ventricular  systole,  and  is 
most  plainly  heard  at  the  spot  where  the  cardiac  impulse 
is  most  readily  felt.     It  is  greatly  altered  in  character  and 


in  BLOOD-PRESSURE  83 

obscured  in  case  of  disease  or  experimental  injury  of  the 
auriculo-ventricular  valves;  but  on  the  other  hand  it  may 
be  heard,  although  modified,  in  a  beating  heart  through 
whose  cavities  the  passage  of  blood  is  temporarily  pre- 
vented. 

The  second  sound  is  without  doubt  caused  by  the  mem- 
branes of  the  semilunar  valves  becoming  tense,  and  thus 
thrown  into  vibrations,  on  their  sudden  closure  at  the  end 
of  the  ventricular  systole.  This^  is  proved  by  the  facts  that 
the  sound  is  loudest  at  a  point  on  the  chest- wall  near  which 
the  semilunar  valves  lie ;  that  it  is  modified  and  obscured 
by  disease  of  these  valves ;  and  that  it  may  be  made  to 
cease  by  experimentally  hooking  back  the  semilunar  valves 
in  a  living  animal. 

12.  Blood-pressure. — When  an  artery  is  cut,  the  out- 
flow of  blood  is  not  uniform  and  smooth,  but  takes  place 
in  jerks  which  correspond  to  each  beat  of  the  heart.  More- 
over, the  blood  spurts  out  with  conside7-able  force,  which, 
although  it  is  greater  at  each  jerk,  is  still  persistent  and 
large  between  the  jerks.  The  obvious  conclusion  to  be 
drawn  from  the  above  observation  is  that  the  blood  in  the 
artery  is  always  under  considerable,  though  variable,  press- 
ure. This  pressure  is  called  biood-pressure.  We  have 
already  explained  how  this  pressure  comes  to  be  established  ; 
but  its  importance  is  so  great  as  a  factor  in  the  circula- 
tion that  we  may  with  advantage  refer  to  this  point  once 
more. 

The  smallest  arteries  and  capillaries  offer  a  considerable 
frictional  resistance  to  the  flow  of  blood  through  them  into 
the  veins,  called,  as  we  have  already  said,  "  peripheral  resist- 
ance." Owing  to  this  resistance,  of  the  total  amount  of 
blood  forced  into  the  arteries  at  each  beat  of  the  heart,  only 
a  portion  can  during  the  actual  beat,  apart  from  the  pause 


84  ELEMENTARY   PHYSIOLOGY  less. 

between  it  and  the  next  beat,  pass  on  into  the  veins.  The 
remainder  is  lodged  in  the  arteries,  whose  walls,  being  dis- 
tensible, axe  put  on  the  stretch  by  the  pressure  of  the  blood 
thrust  into  them  at  each  stroke  of  the  heart,  and  this  press- 
ure of  the  blood  on  the  arterial  wall  is  what  we  mean 
by  "  blood-pressure."  As  soon  as  the  arterial  walls  are 
stretched  their  elastic  properties  come  into  play ;  they 
recoil  and  press  on  the  blood  with  a  force  equal  to  that 
which  puts  them  on  the  stretch.  This  elastic  recoil  squeezes 
the  blood  on  in  the  intervals  between  the  successive  beats 
of  the  heart,  and  thus  renders  the  circulation  continuous. 
In  short,  the  whole  arterial  system  is  always  in  a  state  of 
distension ;  the  work  of  the  heart  consists  in  keeping  up 
this  distended  condition  by  thrusting  fresh  blood  into  the 
arteries  under  pressure ;  and  the  pressure  thus  established 
forces  the  blood  through  the  capillaries,  on  through  the 
veins,  and  so  back  to  the  heart. 

Blood-pressure  is  greatest  in  the  large  arteries  near  the 
heart  and  diminishes  gradually  along  the  arterial  system 
until  we  come  to  the  smallest  arteries  and  capillaries ;  here 
the  pressure  falls  suddenly.  The  sudden  fall  of  pressure  is 
due  to  the  existence  of  what  we  have  already  referred  to  as 
"peripheral  resistance."  This  resistance  must  be  overcome 
in  order  to  drive  the  blood  on  into  the  veins ;  to  overcome 
a  resistance  work  must  be  done,  and  to  do  work,  force  must 
be  employed  and  energy  expended.  Now  blood-pressure  is 
the  force  available  for  overcoming  the  resistance,  and  if  it 
be  thus  used  up  there  is  less  of  it  left,  or,  in  other  words, 
the  pressure  falls.  In  the  veins  the  blood-pressure  is  still 
less  than  in  the  capillaries,  and  diminishes  gradually  along 
their  course  towards  the  heart. 

These  differences  of  pressure  in  the  several  parts  of 
the  vascular  system  determine  the  flow  of  blood  along  the 


in  THE   PULSE  85 

vessels  ;  the  blood  is  always  flowing  from  a  higher  to  a 
lower  pressure ;  the  main,  immediate  work  of  the  heart  is 
to  establish  the  large  blood-pressure  existing  in  the  larger 
arteries. 

When  a  vein  is  cut,  the  blood  does  not  spurt  out  as  it 
does  from  a  cut  artery,  but  oozes  or  trickles  out  gently,  the 
reason  being  that  the  pressure  in  the  veins  is  small.  Fur- 
ther, the  flow  is  in  this  case  continuous  and  not  jerky  as  it 
is  from  a  cut  artery,  in  correspondence  with  the  fact  that 
there  is  no  pulse  in  the  veins  as  there  is  in  the  arteries. 
But  this  statement  requires  that  we  should  next  consider 
the  nature  and  causes  of  the  pulse. 

13.  The  Pulse.  —  If  the  finger  be  placed  on  an  artery 
which  lies  near  the  surface  of  the  body,  such  as  the  radial 
artery  at  the  wrist,  what  is  known  as  the  pulse  will  be  felt 
as  a  slight  throbbing  pressure  on  the  finger,  coming  and 
going  at  regular  intervals  which  correspond  to  the  succes- 
sive beats  of  the  heart.  What  is  felt  is  in  reality  the  in- 
termittent rise  and  fall  of  that  piece  of  the  arterial  wall 
which  lies  immediately  under  the  finger.  This  fact  may 
be  easily  proved  by  placing  a  light  lever  so  as  to  rest  over 
the  artery,  whereupon  its  end  may  be  seen  to  rise  and  fall 
at  the  same  regular  intervals.  This  movement  of  the  arte- 
rial wall  is  due  to  that  distension  of  the  arteries  of  which 
we  have  already  spoken,  which  is  started  at  each  beat  of 
the  heart  by  the  extra  quantity  of  blood  driven  into  them 
by  the  ventricle,  then  travels  in  the  form  of  a  wave  from 
the  larger  to  the  smaller  arteries,  and  corresponds  to  the 
jerky  outflow  of  blood  from  a  cut  artery. 

The  pulse  which  is  felt  by  the  finger  does  not  correspond 
in  time  precisely  with  the  beat  of  the  heart,  but  takes  place 
a  little  after  it,  and  the  delay  is  longer,  the  greater  the  dis- 
tance of  the  artery  from  the  heart.     For  example,  the  pulse 


86  ELEMENTARY   PHYSIOLOGY  less. 

in  the  tibial  artery  on  the  inner  side  of  the  ankle  is  a  little 
later  than  the  pulse  in  the  temporal  artery  in  the  temple. 
By  suitable  instruments  the  rate  at  which  the  pulse  travels 
along  the  arteries  may  be  readily  determined  and  is  found 
to  be  nearly  30  feet  per  second.  This  rate  of  progression 
of  the  pulse-wave  must  be  carefully  distinguished  from  the 
rate  at  which  the  blood  is  flowing  along  in  the  artery.  Even 
in  the  aorta,  where  the  blood  flows  most  rapidly  (p.  89),  its 
velocity  is  not  more  than  about  15  inches  per  second.  In 
fact,  "  the  pulse-wave  travels  over  the  moving  blood  some- 
what as  a  rapidly-moving  natural  wave  travels  along  a 
sluggishly-flowing  river." 

Under  ordinary  circumstances,  the  pulse  is  no  longer  to 
be  detected  in  the  capillaries  or  in  the  veins.  Sometimes 
a  backward  pulse  from  the  heart  along  the  great  venous 
trunks  may  be  observed ;  but  this  is  quite  another  matter, 
and  is  the  result  of  the  movements  of  breathing.  (See 
p.  190.)  The  actual  loss,  or  rather  transformation,  of 
the  pulse  in  the  small  vessels,  is  effected  by  means  of  the 
elasticity  of  the  arterial  walls,  called  into  play  by  the  periphe- 
ral resistance,  in  the  following  manner. 

In  the  first  place  it  must  be  borne  in  mind  that,  owing 
to  the  minute  size  of  the  small  arteries  and  capillaries,  the 
amount  of  friction  taking  place  in  their  channels  when  the 
blood  is  passing  through  them  is  very  great ;  in  other  words, 
they  offer  a  very  great  resistance  to  the  passage  of  the  blood. 
The  consequence  of  this  is  that,  in  spite  of  the  fact  that  the 
total  area  of  the  capillaries  is  so  much  greater  than  that  of 
the  aorta,  the  blood  has  a  difficulty  in  getting  through  the 
capillaries  into  the  veins  as  fast  as  it  is  thrown  into  the 
arteries  by  the  heart.  The  whole  arterial  system,  therefore, 
becomes  over-distended  with  blood. 

Now  we  know  by  experiment  that,  under  such  conditions  as 


HI  THE   PULSE  87 

these,  an  elastic  tube  has  the  power,  if  long  enough  and  elastic 
enough,  to  change  a  jerked  impulse  into  a  continuous  flow. 

If  an  ordinary  syringe  or  other  convenient  form  of  pump 
be  fastened  to  one  end  of  a  long  glass  tube,  and  water  be 
forced  through  the  tube,  it  will  flow  from  the  far  end  in 
jerks,  corresponding  to  the  jerks  of  the  syringe.  This  will 
be  the  case  whether  the  tube  be  quite  open  at  the  far  end, 
or  drawn  out  to  a  fine  point  so  as  to  offer  great  resistance 
to  the  outflow  of  the  water.  The  glass  tube  is  a  rigid  tube, 
and  there  is  no  elasticity  to  be  brought  into  play. 

If  now  a  long  india-rubber  tube  be  substituted  for  the 
glass  tube,  it  will  be  found  to  act  differently,  according  as 
the  opening  at  the  far  end  is  wide  or  narrow.  If  it  is  wide, 
the  water  flows  out  in  jerks,  nearly  as  distinct  as  those 
from  the  glass  tube.  There  is  little  resistance  to  the  out- 
flow, little  distension  of  the  india-rubber  tube,  little  elas- 
ticity brought  into  play.  If,  however,  the  opening  be 
narrowed,  as  by  fastening  to  it  a  glass  tube  drawn  out  to 
a  fine  point,  or  if  a  piece  of  sponge  be  thrust  into  the  end 
of  the  tube  —  if,  in  fact,  in  anyway  resistance  be  offered 
to  the  outflow  of  the  water,  the  tube  becomes  distended, 
its  elasticity  is  brought  into  play,  and  the  water  flows  out 
from  the  end,  not  in  jerks  but  in  a  stream,  which  is  more 
and  more  completely  continuous  the  longer  and  more  elas- 
tic the  tube,  and  the  greater  the  resistance  at  its  open  end. 

Substitute  for  the  syringe  the  heart,  for  the  finely-drawn 
glass  tube  or  sponge  the  small  arteries  and  capillaries,  for 
the  india-rubber  tube  the  whole  arterial  system,  and  you 
have  exactly  the  same  result  in  the  living  body.  Through 
the  action  of  the  elastic  arterial  walls,  the  separate  jets 
from  the  heart  are  blended  into  one  continuous  stream. 
The  whole  force  of  each  blow  of  the  heart  is  not  at  once 
spent  in  driving  a  quantity  of  blood  through  the  capillaries ; 


88  ELEMENTARY   PHYSIOLOGY  less 

a  part  only  is  thus  spent,  the  rest  goes  to  distend  the  elas- 
tic arteries.  But  during  the  interval  between  that  beat  and 
the  next,  the  distended  arteries  are  narrowing  again,  by  vir- 
tue of  their  elasticity,  and  so  are  pressing  the  blood  on  into 
the  capillaries  with  a  force  equal  to  that  by  which  they  were 
themselves  distended  by  the  heart.  Then  comes  another 
beat,  and  the  same  process  is  repeated.  At  each  stroke 
the  elastic  arteries  shelter  the  capillaries  from  part  of  the 
sudden  blow,  and  then  quietly  and  steadily  pass  on  that  part 
of  the  blow  to  the  capillaries  during  the  interval  between 
the  strokes. 

The  larger  the  amount  of  elastic  arterial  wall  thus  brought 
into  play,  i.e.  the  greater  the  distance  from  the  heart,  the 
greater  is  the  fraction  of  each  heart's  stroke  which  is  thus 
converted  into  a  steady  elastic  pressure  between  the  beats. 
Thus  the  pulse  becomes  less  and  less  marked  the  farther 
you  go  from  the  heart ;  any  given  length  of  the  arterial  sys- 
tem, so  to  speak,  being  sheltered  by  the  lengths  between  it 
and  the  heart. 

Every  inch  of  the  arterial  system  may,  in  fact,  be  consid- 
ered as  converting  a  small  fraction  of  the  heart's  jerk  into  a 
steady  pressure,  and  when  all  these  fractions  are  summed  up 
together  in  the  total  length  of  the  arterial  system  no  trace  of 
the  jerk  is  left. 

As  the  immediate,  sudden  effect  of  each  systole  becomes 
diminished  in  the  smaller  vessels  by  the  causes  above  men- 
tioned, the  influence  of  this  constant  pressure  becomes  more 
obvious,  and  gives  rise  to  a  steady  passage  of  the  fluid  from 
the  arteries  towards  the  veins.  In  this  way,  in  fact,  the  arte- 
ries perform  the  same  functions  as  the  air-reservoir  of  a  fire- 
engine,  which  converts  the  jerking  impulse  given  by  the 
pumps  into  the  steady  flow  from  the  nozzle  of  the  delivery 
hose. 


Ill  THE   RATE  OF   BLOOD    FLOW  89 

The  phenomena  so  far  described  are  the  direct  outcome 
of  the  mechanical  conditions  of  the  organs  of  the  circulation 
combined  with  the  rhythmical  activity  of  the  heart.  This 
activity  drives  the  fluid  contained  in  these  organs  out  of  the 
heart  into  the  arteries,  thence  to  the  capillaries,  and  from 
them  through  the  veins  back  to  the  heart.  And  in  the 
course  of  these  operations  it  gives  rise,  incidentally,  to  the 
cardiac  impulse,  the  sounds  of  the  heart,  blood-pressure,  and 
the  pulse. 

14.  The  Rate  of  Blood  Flow.  —  It  has  been  found,  by 
experiment,  that  in  the  horse  it  takes  about  half  a  minute 
for  any  substance,  as,  for  instance,  a  chemical  body,  whose 
presence  in  the  blood  can  easily  be  recognised,  to  complete 
the  circuit,  e.g.  to  pass  from  the  jugular  vein  down  through 
the  right  side  of  the  heart,  the  lungs,  the  left  side  of  the 
heart,  up  through  the  arteries  of  the  head  and  neck,  and  so 
back  to  the  jugular  vein. 

The  greater  portion  of  this  half  minute  is  taken  up  by  the 
passage  through  the  capillaries,  where  the  blood  moves,  it  is 
estimated,  at  the  rate  only  of  about  one  and  a  half  inches  in 
a  minute,  whereas  through  the  the  carotid  artery  of  a  dog  it 
flies  along  at  the  rate  of  about  twelve  inches  in  a  second. 
Of  course,  to  complete  the  circuit  of  the  circulation,  a  blood- 
corpuscle  need  not  have  to  go  through  so  much  as  half  of  an 
inch  of  capillaries  in  either  the  lungs  or  any  of  the  tissues  of 
the  body. 

Inasmuch  as  the  force  which  drives  the  blood  on  is  (put- 
ting the  other  comparatively  slight  helps  on  one  side)  the 
beat  of  the  heart  and  that  alone,  however  much  it  may  be 
modified,  as  we  have  seen,  in  character,  it  is  obvious  that 
die  velocity  with  which  the  blood  moves  must  be  greatest  in 
the  aorta  and  must  diminish  towards  the  capillaries. 

For  with  each  branching  of  the  arteries  the  total  area  of 


go  ELEMENTARY   PHYSIOLOGY  less. 

the  arterial  system  is  increased,  the  total  width  of  the  capil- 
lary tubes  if  they  were  all  put  together  side  by  side  being 
very  much  greater  than  that  of  the  aorta.  Hence  the  blood, 
or  a  corpuscle,  for  instance,  of  the  blood,  being  driven  by 
the  same  force,  viz.  the  heart's  beat,  over  the  whole  body, 
must  pass  much  more  rapidly  through  the  aorta  than  through 
the  capillary  system  or  any  part  of  that  system. 

It  is  not  that  the  greater  friction  in  any  capillary  causes 
the  blood  to  flow  more  slowly  there  and  there  only.  The 
resistance  caused  by  the  friction  in  the  capillaries  is  thrown 
back  upon  the  aorta,  which  indeed  feels  the  resistance  of 
the  whole  vascular  system;  and  it  is  this  total  resistance 
which  has  to  be  overcome  by  the  heart  before  the  blood 
can  move  on  at  all. 

The  blood  driven  everywhere  by  the  same  force  simply 
moves  more  and  more  slowly  as  it  passes  into  wider  and 
wider  channels.  When  it  is  in  the  capillaries  it  is  slowest ; 
after  escaping  from  the  capillaries,  as  the  veins  unite  into 
larger  and  larger  trunks,  and  hence  as  the  total  venous  area 
is  getting  less  and  less,  the  blood  moves  again  faster  and 
faster  for  just  the  same  reason  that  in  the  arteries  it  moved 
slower  and  slower.  It  is,  in  fact,  the  differences  in  the  width 
of  the  "bed"  and  these  alone,  which  determine  the  differ- 
ences in  the  rate  of  flow  at  the  various  points  of  the  vascular 
system. 

A  very  similar  case  is  that  of  a  river  widening  out  in  a 
plain  into  a  lake  and  then  contracting  into  a  narrow  stream 
again.  The  water  is  driven  by  one  force  throughout  (that 
of  gravity).  The  current  is  much  slower  in  the  lake  than 
in  the  narrower  river  either  before  or  behind. 

15.  The  Nervous  Control  of  the  Arteries.  Vaso-motor 
Nerves. — The  arteries,  as  we  have  seen,  are  characterised 
structurally  by  being  elastic  and   muscular.     In  the   large 


in  VASOMOTOR   NERVES  91 

arteries  the  elastic  properties  are  more  marked  than  the 
muscular,  whereas  in  the  smaller  arteries  the  muscular  tissue 
is  present  in  large  amount  relatively  to  the  elastic  elements; 
and  we  have  dealt  in  detail  with  the  significance  of  arterial 
elasticity  and  its  use  in  connection  with  the  establishment  of 
blood-pressure  and  the  disappearance  of  the  pulse.  It  has 
also  been  pointed  out  (p.  58)  that  the  small  arteries  may  be 
directly  affected  by  the  nervous  system,  which  controls  the 
state  of  contraction  of  their  walls,  and  regulates  their  calibre, 
and  thus  governs  the  supply  of  blood  to  each  part  of  the 
body  according  to  its  varying  needs.  The  control  of  the. 
nervous  system  over  the  circulation  in  particular  spots  is  of 
such  paramount  importance  that  we  must  now  deal  with  this 
also  in  some  detail. 

A  phenomenon  with  which  every  one  is  more  or  less  famil- 
iar, either  as  experienced  on  himself  or  observed  on  other 
persons,  is  that  known  as  blushing.  Now  blushing  is  a 
purely  local  modification  of  the  circulation,  and  it  will  be 
instructive  to  consider  how  a  blush  is  brought  about.  An 
emotion,  sometimes  pleasurable,  sometimes  painful,  takes 
possession  of  the  mind ;  thereupon  a  hot  flush  is  felt,  the 
skin  grows  red,  and  according  to  the  intensity  of  the  emo- 
tion these  changes  are  confined  to  the  cheeks  only,  or  extend 
to  the  "  roots  of  the  hair,"  or  "  all  over." 

What  is  the  cause  of  these  changes  ?  The  blood  is  a  red 
and  a  hot  fluid ;  the  skin  reddens  and  grows  hot,  because 
its  vessels  contain  an  increased  quantity  of  this  red  and  hot 
fluid  :  and  its  vessels  contain  more,  because  the  small  arte- 
ries suddenly  dilate,  the  natural  moderate  contraction  of 
their  muscles  being  superseded  by  a  state  of  relaxation  ;  and 
this  relaxation  comes  on  because  the  action  of  the  nervous 
system  which  previously  kept  the  muscles  in  a  state  of  mod- 
erate contraction  is,  for  the  time,  suspended. 


92  ELEMENTARY   PHYSIOLOGY  LESi 

On  the  other  hand,  in  many  people,  extreme  terror  or 
rage  causes  the  skin  to  grow  cold,  and  the  face  to  appear 
pale  and  pinched.  Under  these  circumstances,  in  fact,  the 
supply  of  blood  to  the  skin  is  greatly  diminished,  in  conse- 
quence of  an  increased  contraction  of  the  muscles  of  the 
small  arteries  whereby  these  become  unduly  narrowed  or 
constricted,  and  thus  allow  only  a  small  quantity  of  blood  to 
pass  through  them  ;  and  this  increased  contraction  of  the 
muscular  coats  of  the  arteries  is  brought  about  by  the 
increased  action  of  the  nervous  system.1 

That  this  is  the  real  state  of  the  case  may  be  proved 
experimentally  upon  rabbits.  These  animals  may  be  made 
to  blush  artificially.  If,  in  a  rabbit,  the  sympathetic  nerve 
(Fig.  33,  Sy.),  which  sends  branches  to  the  vessels  of  the 
head,  is  cut,  the  ear  of  the  rabbit,  which  is  covered  by  so 
delicate  an  integument  that  the  changes  in  its  vessels  can 
be  readily  perceived,  at  once  blushes.  That  is  to  say,  the 
vessels  dilate,  fill  with  blood,  and  the  ear  becomes  red  and  hot. 
The  reason  of  this  is  that,  when  the  sympathetic  is  cut,  the 
nervous  impulse  which  is  ordinarily  sent  along  its  branches 
is  interrupted,  and  the  muscles  of  the  small  vessels,  which  were 
previously  slightly  contracted,  become  altogether  relaxed. 

And  it  is  quite  possible  to  produce  pallor  and  cold  in  the 
rabbit's  ear.  To  do  this  it  is  only  necessary  to  irritate  the 
cut  end  of  the  sympathetic  which  remains  connected  with 
the  vessels.  The  nerve  then  becomes  excited,  so  that  the 
muscular  fibres  of  the  vessels  are  thrown  into  a  violent  state 
of  contraction,  which  diminishes  their  calibre  so  much  that 
the  blood  can  hardly  make  its  way  through  them.  Conse- 
quently, the  ear  becomes  pale  and  cold. 

1  Sudden  paleness  is  perhaps  most  frequently  due  to  a  failure  or  stop- 
page of  the  heart's  beat,  as  in  fainting.  But  it  may  also  be  observed  when 
there  is  no  change  in  the  beat  of  the  heart. 


in  VASO-MOTOR   NERVES  93 

This  experiment  on  the  blood-vessels  of  the  rabbit's  ear 
is  of  fundamental  importance  as  proof  of  the  existence  of 
nerves  which  control  locally  the  muscular  elements  of  the 
walls  of  the  smaller  arteries ;  and,  inasmuch  as  this  control 
consists  in  causing  movements  of  the  walls  of  the  vessels,  by 
means  of  which  their  calibre  is  regulated,  the  nerves  which 
exert  the  control  receive  the  general  name  of  vaso-motor 
nerves.  But  from  the  fact  that,  when  the  cut  end  of  the 
sympathetic  nerve  is  irritated,  or,  as  the  physiologist  says, 
is  "  stimulated,"  the  muscular  walls  of  the  arteries  with  which 
it  is  connected  are  always  contracted  and  the  vessels  them- 
selves constricted,  the  sympathetic  is  more  precisely  charac- 
terised as  a  vaso-constrictor  nerve.  Further,  since  merely 
cutting  the  sympathetic  leads  to  a  dilation  of  the  blood- 
vessels of  the  ear,  we  are  justified  in  assuming  that  vaso- 
constrictor impulses  are  continually  being  sent  out  along  this 
nerve,  whereby  the  arteries  are  kept  continually  in  a  condi- 
tion of  slight  or  medium  constriction.  To  this  condition 
the  name  is  given  of  arterial  "  tone."  Now  this  "  tone  "  is 
of  great  importance,  for  by  its  existence  it  at  once  becomes 
possible  to  increase  the  blood-supply  to  any  part  of  the 
body,  as  well  as  to  diminish  it.  Did  the  arteries  possess  no 
"  tone  "  they  would,  under  ordinary  resting  conditions,  be 
dilated  to  their  full  extent,  and  the  part  or  organ  they  sup- 
ply with  blood  would  be  receiving  a  maximum  supply  when 
at  rest.  But  the  organs  of  the  body  are  never  at  rest  for 
long,  and  when  they  become  active  they  require  an  increased 
amount  of  blood,  which  could  not  be  supplied,  at  least  by  a 
vaso-constrictor  mechanism,  but  for  the  existence  of  this 
arterial  tone.  It  would  of  course  be  possible  to  increase  the 
blood-supply  by  means  of  an  increased  activity  of  the  heart ; 
but  this  would  affect  the  supply  to  every  part  of  the  body  at 
the  same  time,  and  what  is  really  wanted  is  a  localised  vari' 


94  ELEMENTARY   PHYSIOLOGY  less. 

ation  in  supply  to  meet  the  varying  needs  of  each  part  or 
organ.  Thus  the  vaso-constrictor  nerves  act  by  carrying 
more  or  less  of  the  same  kind  of  impulse,  leading  to  increase 
or  decrease  of  tone  and  hence  lessened  or  increased  blood- 
supply. 

We  have  quoted  blushing  as  being  a  characteristic  and 
familiar  instance  of  the  action  of  vaso-motor  (vaso-constric- 
tor) nerves.  But  other  examples  of  exactly  similar  action 
are  met  with  throughout  the  whole  body.  Thus,  when  a 
muscle  contracts,  or  when  a  salivary  gland  secretes  saliva,  or 
when  the  stomach  is  preparing  to  digest  food,  in  each  case 
the  small  arteries  of  the  muscle,  salivary  gland,  or  stomach, 
dilate  and  so  flush  the  part  with  blood.  The  organ  in  fact 
blushes  ;  and  this  inner  unseen  blushing  is,  like  the  ordinary 
blushing  described  above,  brought  about  by  vaso-motor 
nerves.  We  shall  see  later  on  that  the  temperature  of  the 
body  is  largely  regulated  by  the  supply  of  blood  sent  to  the 
skin  to  be  cooled,  and  this  supply  is  in  turn  regulated  by 
the  vaso-motor  nervous  system.  Indeed,  everywhere,  all 
over  the  body,  the  nervous  system  by  its  vaso-motor  nerves 
is  continually  supervising  and  regulating  the  supply  of  blood, 
sending  now  more,  now  less  blood,  to  this  or  that  part ;  and 
many  diseases,  such  as  those  when  exposure  to  cold  causes 
congestion  or  inflammation,  are  due  to,  or  at  least  associ- 
ated with,  a  disorder  or  failure  of  this  vaso-motor  activity. 
16.  The  Vaso-motor  Centre.  —  The  vaso-constrictor 
nerves,  which,  by  causing  the  varying  contraction  in  the 
muscular  walls  of  the  arteries,  thus  control  the  supply  of 
blood  to  each  region  of  the  body,  can  all  be  traced  back 
to  the  spinal  cord.  They  make  their  exit  from  this  pari 
of  the  central  nervous  system  by  the  anterior  roots  of  the 
spinal  nerves  of  the  middle  part  of  the  cord,  and  after 
passing   through    the   ganglia   of    the    sympathetic    system 


in  THE  VASOMOTOR   CENTRE  95 

(p.  516)  are  distributed  to  their  various  destinations.  The 
impulses  which  these  nerves  convey  to  the  blood-vessels  are 
of  course  received  by  them  from  the  spinal  cord.  This 
being  the  case,  the  interesting  question  arises  as  to  where 
these  impulses  are  generated  before  their  exit  from  the 
cord.  Experiment  shows  that  under  ordinary  circumstances 
they  come  down  the  cord  from  a  point  higher  up,  i.e.  nearer 
the  brain,  than  that  at  which  the  nerves  themselves  pass  off 
from  the  cord.  In  fact  it  has  been  shown  that  they  origi- 
nate in  a  very  limited  portion  of  the  central  nervous  system, 
located  in  that  part  of  it  which  we  shall  describe  in  a  later 
Lesson  (XII.)  as  the  spinal  bulb  or  medulla  oblongata. 
Here,  then,  the  vaso-constrictor  impulses  are  generated,  and 
since  they  are  the  chief  agents  in  determining  the  state  of 
contraction  or  relaxation  of  the  arteries  of  the  body  as  a 
whole,  this  definitely  localised  part  of  the  bulb  has  received 
the  name  of  the  vaso-motor  centre.      (Fig.  33,  V.M.C.) 

The  cause  of  the  phenomenon  of  arterial  "  tone  "  now 
becomes  quite  clear.  The  vaso-motor  centre  continually 
generates  and  sends  out  to  every  part,  or  rather  to  very 
many  parts,  of  the  body,  impulses  which  suffice  to  keep 
the  muscle  fibres  of  the  arteries  supplying  those  parts  in  a 
condition  of  slight  contraction.  When  the  impulses  to  any 
part  are  increased,  the  supply  of  blood  to  that  part  is 
lessened;  when  the  impulses  are  lessened,  the  supply  is 
increased. 

But  if  the  vaso-motor  centre  is  to  be  of  use,  it  must  itself 
be  under  the  influence  of  impulses  which  can  be  made  to 
play  upon  it  in  such  a  way  as  to  determine  those  variations 
in  its  activity  which  are  essential  to  its  adapting  itself  to  the 
varying  needs  of  either  the  body  as  a  whole  or  any  small 
part  of  the  body.  These  impulses  which  govern  the  vaso- 
motor centre  pass  into  it  either  down  from  the  brain  above, 


96  ELEMENTARY   PHYSIOLOGY  less. 

or  up  from  the  spinal  cord  below.  As  an  instance  of  the 
former  case  we  may  refer  once  again  to  "  blushing."  Here 
the  emotion  which  leads  to  the  blush  starts  impulses  in  the 
brain  (Fig.  33,  a.f.),  which  then  pass  down  to  the  vaso- 
motor centre  and  modify  its  activity  so  as  to  lessen  the 
intensity  of  the  impulses  it  sends  to  the  blood-vessels  of  the 
cheeks.  As  an  instance  of  the  second  case  we  may  refer  to 
the  effects  of  heat  and  cold  applied  to  the  body,  as  deter- 
mining those  variations  of  blood-supply  to  the  skin  by  which 
the  temperature  of  the  body  is  so  largely  regulated  (p.  229). 
Here  the  impulses  are  started  in  the  skin  (Fig.  33,  c.f.)  and, 
travelling  along  certain  sensory  nerves,  enter  the  spinal  cord, 
pass  up  to  the  vaso-motor  centre,  and  as  before  lead  to  the 
necessary  changes  in  its  activity. 

17.  Vaso-dilator  Nerves.  —  Our  consideration  of  vaso- 
motor nerves  has  so  far  led  us  to  the  view  that  the  dilation 
or  widening  of  an  artery  which  leads  to  increased  blood- 
supply  is  usually  the  result  of  cutting  off  or  lessening  con- 
strictor impulses  which  were  previously  passing  along  the 
nerves  to  the  arteries.  But  instances  are  met  with  in  the 
body  where  the  dilation  is  produced  in  an  entirely  different 
way.  Thus  there  is  a  certain  nerve  called  the  chorda 
tympani,  a  branch  of  the  facial  or  seventh  cranial  nerve 
(p.  537),  which  runs  to  the  submaxillary  salivary  glands. 
When  this  nerve  is  simply  severed,  no  obvious  effect  is  pro- 
duced on  the  blood-vessels  of  the  gland.  But  if  now  the 
cut  end  connected  with  the  gland  be  stimulated,  the  small 
arteries  at  once  dilate  powerfully,  the  blood-supply  is  enor- 
mously increased,  and  the  gland  becomes  bright  red  and 
flushed.  In  this  case  we  have  to  deal  with  a  vaso-motor 
nerve  whose  typical  behaviour  when  stimulated  is,  speaking 
broadly,  the  exact  opposite  to  that  of  the  vaso-constrictor 
nerves.     It  is,  in  fact,  a  vaso-motor  nerve  such  that  impulses 


VASO-DILATOR   NERVES 


97 


passing  along  it  give  rise  not  to  constriction  but  to  dilation. 
Hence   it    is    spoken   of  as    a    vaso-dilator    nerve.     Other 


Art 


S.Art.-- 


V.M.C. 


Sp.C. 


A.S 


Fig.  33.  —  Diagram  to  illustrate  the  Position  of  the  Vaso-Motor  Centre, 
the  Paths  of  Vaso-Constrictor  Impulses  from  the  Centre  along  the 
Cervical  Sympathetic  Nerve  and  (part  of)  the  Abdominal  Splanchnic, 
and  the  Course  of  Impulses  to  the  Centre  from  the  Brain  and  from 
an  outlying  Part  of  the  Body. 

Sp.C.  spinal  cord;  V.M.C.  vaso-motor  centre  in  spinal  bulb;  Art.  artery  of  ear; 
S.Art.  subclavian  artery;  Sy.  sympathetic  nervous  system,  the  cervical  part  with  its 
two  ganglia  above  the  subclavian  artery,  the  thoracic  part  with  several  ganglia  below 
the  artery;  A.S.  upper  roots  and  part  of  abdominal  splanchnic  nerve,  which  carries 
vaso-constrictor  fibres  to  the  abdominal  organs.  The  dotted  lines  a.f.  indicate  paths 
of  conduction  for  impulses  to  the  vaso-motor  centre  from  the  brain.  The  dotted  lines 
c.f.  indicate  paths  for  the  passage  of  impulses  to  the  vaso-motor  centre  from  some 
outlying  part  of  the  body  such  as  the  skin.  The  arrows  show  the  directions  in  which 
the  impulses  travel  along  each  path. 

instances  of  the  occurrence  of  similar  vaso-dilator  nerves  are 
met  with,  but,  as  our  knowledge  of  them  is  at  present  uncer 


98  ELEMENTARY   PHYSIOLOGY  less. 

tain  and  incomplete,  we  must  be  content  with  having  sim- 
ply drawn  attention  to  their  existence,  and  to  one  striking 
instance  of  their  action.  It  will  be  observed  that  vaso- 
constrictor nerves  lead  to  dilation  only  through  interference 
with  the  vaso-motor  centre  and  tonic  impulses  ;  vaso-dilator 
nerves  bring  about  dilation  directly. 

18.  The  Nervous  Control  of  the  Heart.  Cardiac  Nerves. 
—  The  heart,  as  we  all  know,  is  not  under  the  direct  influ- 
ence of  the  will,  but  every  one  is  no  less  familiar  with  the 
fact  that  the  actions  of  the  heart  are  wonderfully  affected 
by  all  forms  of  emotion.  Men  and  women  often  faint,  and 
have  sometimes  been  killed  by  sudden  and  violent  joy  or 
sorrow  ;  and  when  they  faint  or  die  in  this  way,  they  do  so 
because  the  perturbation  of  the  brain  gives  rise  to  a  some- 
thing which  arrests  the  heart  as  dead  as  you  stop  a  stop- 
watch with  a  spring.  On  the  other  hand,  other  emotions 
cause  that  extreme  rapidity  and  violence  of  action  which  we 
call  palpitation.  These  facts  suggest  at  once  that  the  heart, 
like  the  arteries,  is  subject  to  control  by  the  central  ner- 
vous system,  and  we  must  now  consider  the  more  important 
details  of  this  control. 

The  heart  is  well  supplied  with  nerves.  There  are  many 
small  ganglia,  or  masses  of  nerve  cells,  lodged  in  the  sub- 
stance of  the  heart,  more  especially  in  the  auricles,  and 
nerves  spread  from  these  ganglia  over  the  walls,  both  of  the 
auricles  and  ventricles.  Moreover,  several  nerves  reach  the 
heart  from  the  outside  (Fig.  34).  Of  these  the  most  im- 
portant are  branches  of  a  remarkable  nerve  which  starts 
from  the  spinal  bulb,  and  supplies  not  only  the  heart,  but 
the  lungs,  alimentary  canal,  and  other  parts,  and  which  is 
called  the  pneumogastric,  or,  from  its  wandering  course,  the 
vagus  (p.  538).  Other  nerves  reaching  the  heart  seem  to 
come  from  the  sympathetic  system,  but  may  be  traced  back 


in  THE  NERVES   OF  THE   HEART  99 

through  this  system  to  the  spinal  cord,  and,  for  reasons 
which  will  presently  become  apparent,  are  called  accelera- 
tor nerves. 

The  heart,  as  already  explained  (p.  75),  contracts  rhyth- 
mically, but  the  regular  rhythmical  succession  of  the  ordi- 
nary contractions  is  not  primarily  dependent  upon  the  ganglia 
lodged  in  its  substance,  as  was  at  one  time  supposed  to  be 
the  case.  Neither  does  it  depend  on  the  action  of  the 
nerves  connected  with  the  heart,  since  the  movements  con- 
tinue even  after  the  heart  is  removed  from  the  body. 
Hence  we  must  conclude,  and  experiment  bears  out  the 
conclusion,  that  the  muscular  substance  of  which  the  heart 
is  made  is  itself  endowed  ivith  the  power  of  contracting  and 
relaxing  at  regular  intervals.  On  the  other  hand,  the  influ- 
ences which  alter  the  heart's  action,  as  in  fainting  or  palpi- 
tation, do  as  a  rule  come  to  the  heart  from  without,  and  are 
carried  to  the  heart  along  the  vagus  and  accelerator  nerves. 
This  may  be  demonstrated  on  animals,  such  as  frogs,  with 
great  ease. 

If  a  frog  be  pithed,  or  its  brain  destroyed,  so  as  to  oblit- 
erate all  sensibility,  the  animal  will  continue  to  live,  and  its 
circulation  will  go  on  perfectly  well  for  a  prolonged  period. 
The  body  may  be  laid  open  without  causing  pain  or  other 
disturbance,  and  then  the  heart  will  be  observed  beating 
with  great  regularity.  It  is  possible  to  make  the  heart 
move  a  long  lever  backwards  and  forwards  ;  and  if  frog  and 
lever  are  covered  with  a  glass  shade,  the  air  under  which  is 
kept  moist,  the  lever  may  vibrate  with  great  steadiness  for 
a  couple  of  days. 

It  is  easy  to  adjust  to  the  frog  thus  prepared  a  contri- 
vance by  which  electrical  shocks  may  be  sent  through  the 
vagus  nerves,  so  as  to  stimulate  them.  If  the  stimulation  is 
only  gentle  or  weak,  the  heart  will  be  seen  to  beat  more 


ioo  ELEMENTARY    PHYSIOLOGY  less. 

slowly,  and  at  the  same  time  each  beat  is  rather  more  feeble, 
as  shown  by  the  diminished  distance  over  which  the  end  of 
the  lever  moves.  But  if  the  stimulation  is  strong,  the  lever 
almost  immediately  stops  dead,  and  the  heart  will  be  found 
quiescent,  with  relaxed  and  distended  walls.  After  a  little 
time  the  influence  of  the  vagus  passes  off,  the  heart  recom- 
mences its  work  as  vigorously  as  before,  and  the  lever  vi- 
brates through  the  same  arc  as  formerly.  With  careful 
management,  this  experiment  may  be  repeated  Very  many 
times ;  and  after  every  arrest  by  the  stimulation  of  the 
vagus,  the  heart  resumes  its  work. 

If,  on  the  other  hand,  the  stimulation  be  applied  to  the 
sympathetic  nerves,  then  an  effect  is  produced  which  is 
exactly  the  opposite  to  that  which  results  from  stimulating 
the  vagus.  The  lever  moves  more  rapidly  and  over  a 
greater  distance,  showing  quite  clearly  that  the  heart  is  now 
beating  faster  and  that  each  beat  is  stronger. 

No  clearer  proof  could  be  desired  than  is  afforded  by  the 
above  experiments,  that  the  heart  of  the  frog  is  controlled 
by  two  antagonistic  nerves,  of  which  one,  the  vagus,  carries 
impulses  which  slow  and  finally  stop  its  beat,  while  the 
other,  the  accelerator,  conveys  impulses  which  make  it  beat 
faster.  Since  there  is  no  reason  for  supposing  that  the 
working  mechanisms  of  a  frog's  heart  differ  in  any  essential 
way  from  those  of  the  mammalian  heart,  we  may  at  once 
apply  these  striking  results  to  the  human  heart.  It  is,  in 
fact,  recorded  of  a  certain  well-known  physiologist,  that, 
having  a  small  hard  tumour  in  his  neck,  in  close  proximity 
to  the  vagus  nerve,  he  could  press  the  vagus  against  this 
tumour  and  by  thus  stimulating  the  nerve  mechanically 
cause  a  stoppage  of  his  own  heart-beat. 

The  heart,  then,  is  controlled  by  two  kinds  of  antagonistic 
influences,  analogous  to  those  previously  described  as  con- 


in  THE   CARDIO-INHIBITOKY    CENTRE  101 

trolling  the  muscular  walls  of  the  arteries.  Moreover,  both 
the  cardiac  nerves  are  connected  with  the  central  nervous 
system,  the  one  coming  from  the  spinal  bulb,  the  other  from 
the  spinal  cord,  so  that  the  influences  they  convey  to  the 
heart  must,  as  in  the  case  of  the  vaso-motor  nerves,  origi- 
nate in  the  central  nervous  system  (Fig.  34).  We  saw,  how- 
ever (p.  94),  that  the  impulses  carried  by' the  vaso-motor 
nerves  are  generated  in  a  very  specially  localised  part  of  the 
spinal  bulb,  and  the  interesting  question  at  once  arises  :  Is 
there  a  similarly  localised  centre  in  which  the  impulses 
which  modify  the  beat  of  the  heart  take  their  origin?  The 
answer  to  this  question  is  in  the  affirmative,  for  experiment 
shows  that  the  impulses  which,  travelling  along  the  vagus, 
can  stop  or,  as  the  physiologist  says,  "inhibit"  the  heart's 
beat,  are  generated  in  a  limited  part  of  the  spinal  bulb,  in 
close  proximity  to  the  vaso-motor  centre.  This  part  is 
therefore  known  as  the  cardio-inhibitory  centre  (Fig.  34, 
C.I.C.).  There  are  reasons  for  supposing  that  this  centre, 
like  the  vaso-motor  centre,  is  continually  at  work  sending 
out  impulses  to  the  heart  along  the  vagus,  which  check  its 
activity,  so  that  in  many  animals  the  heart  beats  more 
quickly  after  the  vagus  nerves  are  cut. 

The  cardio-inhibitory  centre  may,  like  the  vaso-motor 
centre,  be  itself  influenced  by  impulses  which  reach  it  either 
from  the  brain  above  or  the  spinal  cord  below.  In  this  way 
the  heart  is  indirectly  connected  with  all  parts  of  the  body, 
so  that  by  nervous  agencies  its  beat  may  be  made  to  van- 
according  to  the  varying  needs  of  the  body  as  a  whole  or  of 
its  several  parts.  For  instance,  when  taking  exercise,  the 
restraining  influence  of  the  centre  is  lessened  and  the  heart 
beats  faster,  thus  providing  for  an  increased  rapidity  of  the 
circulation  to  meet  the  demands  of  the  more  actively  con- 
tracting muscles.     It  is,  of  course,  possible   that  the  faster 


ELEMENTARY   PHYSIOLOGY 


beat  of  the  heart  may  also  be  due  to  impulses  along  the 
accelerator  nerves.  Again,  when  a  person  faints  from  a 
sudden  emotion,  an  influence  is  started  in  the  brain,  passes 


a.f. 


^   k 


feb.Vg 


-C.I.C. 


--Sp.C. 


■**-m.f. 


Fig.  34. —  Diagram  to  illustrate  the  Position  of  the  Cardio-Inhibitorv 
Centre,  the  Paths  of  Inhibitory  and  Accelerator  Impulses  from  the 
Central  Nervous  Svstem  to  the  Heart,  and  the  Course  of  Impulses 
to  the  Centre  from  the  Brain  and  from  an  outlying  Part  of  the  Body. 

Sp.C.  spinal  cord;  C.I.C.  cardie-inhibitory  centre;  V.G.  ganglion  of  the  vagus; 
Vg.  main  trunk  of  the  vagus;  c.b-.Vg.  cardiac  branches  of  vagus,  supplying  the 
heart;  S.Art.  subclavian  artery;  Sy.  sympathetic  nervous  system,  the  cervical  part 
with  its  two  ganglia  above  the  subclavian  artery,  the  thoracic  part  with  several  gan- 
glia below  the  artery,  c.b.Sy.  cardiac  branches  of  the  sympathetic  supplying  the  heart. 
The  dotted  lines  a.f.  indicate  paths  of  conduction  for  impulses  to  the  cardio-inhibitory 
centre  from  the  brain.  The  dotted  lines  m.f.  indicate  paths  for  the  passage  of  im- 
pulses to  the  cardio-inhibitory  centre  from  some  outlying  part  of  the  body  such  as 
the  stomach  or  intestines.  The  arrows  show  the  directions  in  which  the  impulses 
travel  along  each  path.  • 


down  to  the  centre  in  the  spinal  bulb  (Fig.  34,  a.f.),  in- 
creases its  action  and  stops  for  a  lime  the  beating  of  the 
heart.     Or  again,  fainting  may  result  from  a  blow  on  the 


in  THE   CARDIO-INHIBITORY   CENTRE  103 

stomach  ;  in  this  case,  the  influence  starts  at  the  part  struck 
(Fig.  34,  m.f.),  and,  passing  up  the  spinal  cord  to  the 
cardio-inhibitory  centre,  increases  its  activity  and  leads  as 
before  to  stoppage  of  the  heart.  The  rapid  and  violent 
beating  of  the  heart  which  we  speak  of  as  "  palpitation  " 
may,  on  the  other  hand,  be  often  due  to  some  emotion 
which  in  this  case  lessens  the  activity  of  the  centre  and 
hence  diminishes  the  restraint  which  it  ordinarily  exerts 
over  the  heart.  But  of  course  palpitation  may  also,  at 
times,  be  due  to  impulses  reaching  the  heart  along  those 
nerves  which  we  have  described  above  as  the  accelerators. 

Our  knowledge  of  the  existence  and  position  of  the  car- 
dio-inhibitory centre  is  quite  clear  and  definite.  It  is  possi- 
ble that  a  cardio-augmentor  (-accelerator)  centre  may  also 
exist,  but  at  present  we  have  no  exact  knowledge  of  its  exist- 
ence ;  hence  in  the  accompanying  figure  the  accelerator 
nerves  are  shown,  as  originating  in  the  central  nervous  sys- 
tem, but  not  arising  from  any  definitely  localised  centre. 

19.  The  Proofs  of  the  Circulation.  —  The  evidence  that 
the  blood  circulates  in  man,  although  perfectly  conclusive, 
is  almost  all  indirect.  The  most  important  points  in  the 
evidence  are  as  follows :  — 

In  the  first  place,  the  disposition  and  structure  of  the 
organs  of  circulation,  and  more  especially  the  arrangement 
of  the  various  valves,  will  not,  as  was  shown  by  Harvey, 
the  discoverer  of  the  circulation  (1628),  permit  the  blood 
to  flow  in  any  other  direction  than  in  the  one  described 
above.  Moreover,  we  can  easily  with  a  syringe  inject  a 
fluid  from  the "  vena  cava,  for  instance,  through  the  right 
side  of  the  heart,  the  lungs,  the  left  side  of  the  heart,  the 
arteries  and  capillaries,  back  to  the  vena  cava ;  but  not  the 
other  way.  In  the  second  place,  we  know  that  in  the  living 
body  the  blood  is  continually  flowing  in  the  arteries  towards 


104 


ELEMENTARY   PHYSIOLOGY 


the  capillaries,  because  when  an  artery  is  tied,  in  a  living 
body,  it  swells  up  and  pulsates  on  the  side  of  the  ligature 


Fig.  35.  —  Portion  of  the  Web  of  a  Frog's  Foot  seen  under  a  low  Magni- 
fying Power,  the  Blood-vessels  only  being  represented,  except  in 
the  Corner  of  the  Field,  where  in  the  Portion  marked  off  the  Pig- 
ment Spots  are  also  drawn. 

a,  small  arteries;  7',  small  veins;  the  minute  tubes  joining  the  arteries  and  the 
veins  are  the  capillaries.  The  arrows  denote  the  direction  of  the  circulation.  The 
larger  artery  running  straight  up  in  the  middle  line  breaks  up  into  capillaries  at 
points  higher  up  than  can  be  shown  in  the  drawing. 

nearest  the  heart,  whereas  on  the  other  side   it  becomes 
empty,  and  the  tissues  supplied  by  the  artery  become  pale 


?H  THE   PROOFS   OF  THE  CIRCULATION  105 

from  the  want  of  a  supply  of  blood  to  their  capillaries.  And 
when  we  cut  an  artery  the  blood  is  pumped  out  in  jerks 
from  the  cut  end  nearest  the  heart,  whereas  little  or  no 
blood  comes  from  the  other  end.  When,  however,  we  tie  a 
vein  the  state  of  things  is  reversed,  the  swelling  taking  place 
on  the  side  farthest  from  the  heart,  etc.  etc.,  showing  that 
in  the  veins  the  blood  flows  from  the  capillaries  to  the 
heart. 

But  certain  of  the  lower  animals,  the  whole,  or  parts,  of 
the  body  of  which  are  transparent,  readily  afford  direct 
proof  of  the  circulation ;  in  these  the  blood  may  be  seen 
rushing  from  the  arteries  into  the  capillaries,  and  from  the 
capillaries  into  the  veins,  so  long  as  the  animal  is  alive  and 
its  heart  is  at  work.  The  animal  in  which  the  circulation 
can  be  most  conveniently  observed  is  the  frog.  The  web 
between  its  toes  is  very  transparent,  and  the  corpuscles  sus- 
pended in  its  blood  are  so  large  that  they  can  be  readily  seen 
as  they  slip  swiftly  along  with  the  stream  of  blood,  when  the 
toes  are  fastened  out,  and  the  intervening  web  is  examined 
under  a  microscope  (Fig.  35). 

20.  The  Capillary  Circulation.  —  The  essential  charac- 
teristics of  blood-flow  through  the  capillaries  may  also  be 
easily  studied  in  such  a  preparation.  In  the  smallest  capil- 
laries the  corpuscles  pass  along  singly,  sometimes  following 
each  other  in  close  file,  at  other  times  leaving  quite  con- 
siderable gaps  in  their  succession.  Frequently  one  or  more 
corpuscles  may  remain  stationary  for  a  moment  and  then 
pass  on  again.  The  red  corpuscles,  which  in  the  frog  are 
oval  and  comparatively  large,  glide  along  with  their  long 
axis  parallel  to  the  direction  of  the  stream,  and  may  often 
be  observed  to  be  squeezed  out  of  shape  by  pressure  against 
the  wall  of  the  capillary  (Fig.  36,  G  and  H).  In  the  larger 
capillaries,  more  especially  in  mammals  whose  corpuscles  are 


io6 


ELEMENTARY    PHYSIOLOGY 


Fig.  36.  —  Very  small  Portion  of  Fig.  35  very  highly  magnified. 

A,  walls  of  capillaries;  B,  tissue  of  web  lying  between  the  capillaries;  C,  cells  oi 
epidermis  covering  web  (these  are  shown  only  in  the  right  hand  and  lower  part  of  the 
field;  in  the  other  parts  of  the  field  the  focus  of  the  microscope  lies  below  the  epider- 
mis); D,  nuclei  of  these  epidermal  cells;  E,  pigment  cells  contracted,  not  partially 
expanded  as  in  Fig.  3s:  /■',  red  blood-corpuscle  (oval  in  the  frog)  passing  along 
capillary  nucleus  not  visible;  (?,  another  corpuscle  squeezing  its  way  through  a 
capillary,  the  1  anal  "f  which  is  smaller  than  its  own  transverse  diameter;  //,  another 
corpuscle  bending  as  it  slides  round  a  corner;  A",  corpuscle  in  capillary  seen  througli 
the  epidermis;  /,  white  blood-corpuscle. 


in  INFLAMMATION  10} 

smaller  than  in  the  frog,  the  corpuscles  often  pass  along  two 
or  three  abreast.  Further,  in  these  larger  capillaries  it  may 
be  seen  that  the  red  corpuscles  tend  to  keep  to  the  centre 
of  the  stream,  leaving  a  clear  layer  of  fluid  along  the  sides 
of  the  blood-vessels.  This  is  due  to  the  fact  that  the  fluid 
friction  (already  referred  to  on  p.  81)  is  greater  close  to  the 
walls  of  the  capillaries  than  in  the  middle  of  the  stream,  and 
the  corpuscles  pass  along  where  the  resisting  friction  is  least. 
The  colourless  or  "  white  "  corpuscles  usually  move  more 
slowly  and  irregularly  than  the  red,  and  may,  as  a  rule,  be 
seen  to  lie  in  the  clearer  layer  of  fluid  at  the  side  of  the  cur- 
rent. Moreover,  they  frequently  stop  for  an  appreciable 
time,  as  if  sticking  to  the  wall  of  the  capillary,  and  then  roll 
on  again ;  probably  because  they  are  more  adhesive  than 
the  red  corpuscles,  in  harmony  with  their  power  of  executing 
amoeboid  movements  (see  p.  126). 

21.  Inflammation.  —  All  persons  are  more  or  less  familiar 
with  a  peculiar  and  unusual  condition  which  may  arise  in 
almost  any  part  of  the  body,  and  which  they  describe  by 
speaking  of  the  part  as  "inflamed."  To  ordinary  observa- 
tion the  characteristics  of  the  condition  are  that  the  inflamed 
region  becomes  flushed  and  red,  that  it  feels  warmer  than 
usual,  that  it  becomes  swelled  and  painful,  and,  finally,  if  the 
inflammation  is  severe,  that  a  thick  yellowish  fluid  is  formed 
which  is  commonly  known  as  "  matter,"  or  more  correctly 
as  pus.  Such  a  series  of  changes  may  be  observed  during 
the  formation  and  breaking  of  a  boil.  But  the  several  stages 
just  named  are  merely  the  external  evidences  of  changes 
taking  place  at  the  same  time  in  the  minute  blood-vessels 
and  circulation  of  the  part  affected,  and,  since  these  changes 
throw  an  interesting  light  on  the  relations  ordinarily  existing 
between  the  walls  of  the  blood-vessels  and  the  adjacent 
blood,  they  are  worthy  of  a  short  consideration. 


io8  ELEMENTARY   PHYSIOLOGY  less 

If,  when  the  web  of  a  frog's  foot,  or  other  suitably  trans- 
parent part  of  an  animal,  is  adjusted  for  observation  under 
the  microscope,  some  irritant  be  applied  to  it  such  as  a  trace 
of  mustard,1  the  following  events  may  be  readily  observed. 
The  minute  arteries  dilate,  the  blood  flows  faster,  and  the 
increased  quantity  of  blood  forced  through  the  capillaries 
distends  them  so  that  they,  as  well  as  the  smallest  veins, 
appear  to  be  similarly  dilated.  This  accounts  for  the  initial 
greater  redness  and  warmth  of  an  inflamed  part.  Very 
soon  the  colourless  corpuscles  are  seen  to  be  collecting  in 
large  numbers  in  the  clear  layer  of  fluid  next  to  the  walls  of 
the  capillaries  and  veinlets,  and  seem  to  adhere  more  firmly 
than  usual  to  the  walls  of  these  vessels.  Further,  blood 
"platelets"  (see  p.  130),  not  previously  visible,  begin  to 
collect  also  with  and  among  the  white  corpuscles.  Follow- 
ing upon  this  the  stream  of  blood  begins  to  flow  more  slowly 
although  the  blood-vessels  are  still  widely  dilated.  And  now 
a  very  striking  phenomenon  takes  place.  The  white  cor- 
puscles make  their  way  by  amoeboid  movements  through  the 
thin  walls  of  the  capillaries  and  collect  outside  them  in  the 
spaces  in  the  neighbouring  tissue.  At  the  same  time  that 
the  corpuscles  are  in  this  way  "  migrating,"  a  considerable 
quantity  of  the  fluid  part  of  the  blood  also  passes  out  through 
the  walls  of  the  blood-vessels  into  the  adjacent  tissue.  This 
accounts  for  the  characteristic  swelling  of  an  inflamed  part. 
If  the  action  of  the  irritant  is  continued,  more  and  more 
white  corpuscles  collect  in  the  vessels,  the  blood-flow  be- 
comes slower  and  slower,  red  corpuscles  are  arrested  in  large 
numbers  among  the  white,  and  finally  the  circulation  stops 
altogether.  At  this  stage  red  corpuscles  pass  through  the 
walls  of  the  vessels  as  well  as  the  white,  and  the  latter,  multi- 

1  Used  similarly  as  an  irritant  in  the  form  of  the  ordinaiy  domestic  mus- 
tard poultice. 


Ill  THE   LYMPHATIC    SYSTEM  109 

plying  rapidly  in  the  spaces  of  the  tissue  outside  the  blood- 
vessels, and  undergoing  certain  other  slight  changes,  are 
converted  into  pus  corpuscles. 

The  appearances  just  described  seem  to  indicate  that  the 
condition  of  the  walls  of  the  capillaries  (and  of  the  smallest 
veins  and  arteries)  plays  a  very  important  but  as  yet  obscure 
part  in  determining  the  characteristics  of  the  normal  circula- 
tion through  these  passages.  And  since  in  an  inflamed  area 
the  flow  of  blood  becomes  slower  and  slower,  and  ultimately 
ceases,  even  while  the  blood-vessels  are  more  widely  dilated 
than  usual,  the  condition  of  the  walls  of  these  vessels  may 
evidently  play  a  very  important  part  in  determining  varia- 
tions in  that  "  peripheral  resistance "  which,  as  we  have 
previously  explained,  is  of  paramount  importance  to  the 
working  of  the  circulation  throughout  every  part  of  the 
whole  body.  Moreover,  it  is  evident  that  the  condition  of 
the  walls  of  the  capillaries  may  also  at  any  moment  modify 
the  amount  of  the  fluid  part  of  the  blood  which  is  continually 
passing  out  through  those  walls  as  lymph  (p.  no)  for  the 
nutrition  of  the  neighbouring  tissues. 

Part  II.  —  The  Lymphatic  System  and  the  Circulation 
of  Lymph 

1.  The  General  Arrangement  of  the  Lymphatics. — 
Food,  as  we  have  already  pointed  out  (p.  22),  after  diges- 
tion in  the  alimentary  canal,  is  absorbed  into  the  blood-vessels 
and  lacteals  of  that  canal  and  whirled  away  in  the  current 
of  the  circulation  for  distribution  as  nutritive  material  to  all 
parts  of  the  body.  But  we  have  also  drawn  attention  to 
the  fact  (p.  56)  that  the  ultimate  anatomical  components, 
the  cells  and  tissues,  of  every  part  of  the  body  lie  outside  the 
blood-vessels.    It  is  therefore  clear  that  the  tissues  are  every- 


no  ELEMENTARY   PHYSIOLOGY  less. 

where  separated  from  the  blood  by  at  least  the  thickness  of 
the  walls  of  the  vessels,  and  in  any  case  cannot  draw  the 
nutriment  they  require  directly  from  the  blood,  since  they 
are  nowhere  in  direct  contact  with  it.  Neither  can  they,  for 
the  same  reason,  discharge  the  waste  they  are  always  produc- 
ing directly  into  the  blood  for  its  removal  as  a  preliminary 
to  its  excretion.  Both  these  difficulties  are  however  got  over 
by  the  fact  that  a  portion  of  the  fluid  part  of  the  blood  is 
continually  exuding  through  the  walls  of  the  capillaries  into 
the  neighbouring  tissues,  taking  with  it  the  nutriment  neces- 
sary for  each  tissue  and  providing  a  fluid  connection  between 
the  tissue  and  the  blood  across  which  the  waste  from  the 
tissues  can  be  returned  into  the  blood.  The  fluid  which  thus 
exudes  is  called  lymph,1  and  may  be  regarded  as  a  sort  of 
"  middleman  "  between  the  blood  on  the  one  hand  and  the 
tissue  on  the  other.  But  if  this  lymph  is  to  be  thoroughly 
efficient  as  a  nutriment  for  the  tissues  it  should,  presumably, 
contain  more  food  material  than  the  tissues  actually  require 
as  an  average,  and  it  must,  therefore,  be  an  economy  to  the 
body  if  the  lymph,  after  having  served  the  needs  of  the  tis- 
sues, is  gathered  up  again  and  returned  to  the  blood  for 
further  use.  Now  this  is  exactly  what  does  take  place,  and 
the  means  for  ensuring  the  return  of  the  lymph  to  the  blood- 
vessels are  as  follows. 

Besides  the  capillary  network  and  the  trunks  connected 
with  it  which  constitute  the  blood-vascular  system,  all  parts 
of  the  body  which  possess  blood  capillaries  also  contain 
another  set  of  what  are  termed  lymph- capillaries,  mixed  up 
with  those  of  the  blood-vascular  system,  but  not  directly 
communicating  with  them,  and,  in  addition,  differing  from 
the  blood-capillaries  in  being  connected  with  larger  vessels 

1  The  mode  of  formation,  composition,  and  properties  of  lymph  are  dealt 
with  in  Lesson  IV. 


Ill 


THE    LYMPHATIC    SYSTEM 


of  only  one  kind.  That  is  to  say,  they  open  only  into  trunks 
that  carry  fluid  away  from  them  and  thus  bear  the  same 
relationship  to  the  lymph-capillaries  that 
the  veins  do  to  blood-capillaries.  These 
trunks  are  known  as  the  lymphatic  vessels, 
and  further  resemble  the  small  veins  in  the 
general  structure  of  their  walls  and  in  being 
abundantly  provided  with  valves,  similar  to 
those  in  the  veins,  which  freely  allow  of  the 
passage  of  lymph  from  the  lymph-capilla- 
ries, but  obstruct  the  flow  of  any  liquid  in 
the  opposite  direction.  But  the  lymphatic 
vessels  differ  from  the  veins  in  that  they 
do  not  rapidly  unite  into  larger  and  larger 
trunks  which  present  a  continually  increas- 
ing calibre  and  allow  a  flow  without  inter- 
ruption to  the  heart.  On  the  contrary, 
remaining  nearly  of  the  same  size,  they  at 
intervals  become  connected  with  small, 
rounded  or  often  bean-shaped  bodies  called 
lymphatic  glands,  entering  the  glands  at  one 
side  and  emerging  at  the  opposite  side  as  FlG-  37-— The  Lym- 

0  .  PHATICS     OF     THE 

new  lymphatic  vessels  (Fig.  %7,g).  Front    of    the 

J      i  \      5    OltSJ  Right  Arm. 

Sooner  or  later  the  great  majority  of  the       ,       ,  ...     .     , 

0  J  J  g,   lymphatic   glands, 

smaller   lymphatic  vessels  pour  their  con-     °"  ,the  course  of 

J       l  x  the  lymphatics. 

tents  into  a  tube  which  is  about  as  large 
as  a  goose-quill,  lies  in  front  of  the  backbone,  and  is 
called  the  thoracic  duct.  This  opens  at  the  root  of  the 
neck  into  the  conjoined  trunks  of  the  great  veins  (jugular 
and  subclavian)  which  bring  back  the  blood  from  the  left 
side  of  the  head  and  the  left  arm.     (Fig.  38,/,  g.) 

The  remaining  lymphatics,  chiefly  those  of  the  right  side 
of  the  head  and  neck,  the  right  arm  and  right  lung,  are  con- 


ii2  ELEMENTARY   PHYSIOLOGY  less, 

nected  by  a  common  canal  with  the  corresponding  vein  of 
the  right  side. 

The  lower  part  of  the  thoracic  duct  is  dilated,  and  is 
called  the  receptacle  of  the  chyle  (Fig.  38,  a).  This  part 
receives  more  particularly  the  lymphatics  from  the  intestines, 
which,  though  they  differ  in  no  essential  respect  from  other 
lymphatics,  are  called  lacteals,  because,  after  a  meal  con- 
taining much  fatty  matter,  they  are  filled  with  a  milky  fluid 
termed  chyle.  The  lacteals,  or  lymphatics  of  the  small 
intestine,  not  only  form  networks  in  its  walls,  but  send  blind 
prolongations  into  the  little  processes  termed  villi,  with 
which  the  mucous  membrane  of  that  intestine  is  beset. 
(P.  280.) 

Where  the  two  principal  trunks  of  the  lymphatic  system 
open  into  the  veins,  valves  are  placed,  which  allow  of  the 
passage  of  fluid  in  one  direction  only,  namely  from  the  lym- 
phatic to  the  veins,  the  blood  in  the  veins  being  unable  to 
get  into  the  lymphatics,  and  in  this  way  the  lymph  from 
every  part  of  the  body  is  collected  and  returned  into  the 
blood. 

2.  The  Origin  and  Structure  of  Lymphatics. — The 
cells  of  which  the  tissues  of  the  body  are  built  up,  though 
lying  closely  applied  to  each  other,  are  often  separated  by 
extremely  minute  spaces.  These  spaces  are  particularly 
plentiful  in  that  form  of  connective  tissue  called  "areolar." 
As  has  been  seen  (p.  49),  it  is  made  up  of  bundles  of  fine 
threads  or  fibres  which  cross  one  another  in  all  directions 
and  thus  form  a  sort  of  feltwork  of  interlacing  fibres.  Some 
of  the  spaces  in  this  tissue  are  comparatively  large  and  are 
called  areolce,  whence  the  name  areolar  tissue.  This  tissue 
is,  as  we  have  said  (p.  11),  present  in  every  part  of  the 
body,  and  of  course  supports  the  blood-capillaries,  which 
are  thus,  in  reality,  merely  minute  tubes  lying  imbedded  in 


HI 


THE  THORACIC    DUCT 


"3 


connective  tissue.     The  chinks  and  spaces  of  the  tissues  are 
filled  with  that  fluid  exudation  from  the  blood-vessels  and 


Fig.  38.  —The  Thoracic  Duct. 

The  thoracic  duct  occupies  the  middle  of  the  figure.  It  lies  upon  the  spinal 
column,  at  the  sides  of  which  are  seen  portions  of  the  ribs  (/). 

a,  the  receptacle  of  the  chyle;  i,  the  trunk  of  the  thoracic  duct,  opening  at  c  into 
the  junction  of  the  left  jugular  (_/)  and  subclavian  {g)  veins  as  they  unite  into  the 
left  innominate  vein,  which  has  been  cut  across  to  show  the  thoracic  duct  running 
behind  it;  if,  lymphatic  glands  placed  in  the  lumbar  regions;  h,  the  superior  vena 
cava,  formed  by  the  junction  of  the  right  and  left  innominate  veins. 


114  ELEMENTARY    PHYSIOLOGY  less. 

tissue  elements  already  spoken  of  as  lymph,  and  hence  are 
themselves  often  called  lymph-spaces.  In  these  lymph-spaces 
we  see  the  origin  or  beginning  of  the  lymphatic  system. 

From  the  lymph-spaces  the  lymph  passes  directly  into 
the  lymph- capillaries  (Fig.  39).  These  are  also  essentially 
spaces  in  the  mesh  work  of  connective  tissue,  but  they  are 
now  lined  by  a  single  layer  of  extremely  thin,  flat,  nucleated, 
epithelial  cells,  very  similar  to  those  composing  the  wall  of 


Fig.  39. — Origin  of  Lymphatics.     (After  Landois.) 
S,  lymph-spaces  opening  directly  into  lymphatic  capillary;   A  ,  lymph-spaces  unil- 
ing  to  form  a  lymphatic  capillary:   E,  epithelial  cells  forming  walls  of  capillnry. 

a  blood-capillary.  These  cells  are  joined  to  each  other  by 
their  edges  so  that  they  form  a  system  of  minute  tubes, 
larger  than  blood-capillaries  and  wandering  more  irregularly. 
The  lymphatic  vessels,  into  which  the  lymph-capillaries 
pour  their  contents  on  the  way  towards  the  thoracic  duct, 
possess    a  structure  essentially  similar   to    that    of   a    vein 


LSI  LYMPHATIC   GLANDS  115 

(P-  59)  >  but  they  differ  from  a  vein  in  that  their  walls  are 
thinner,  so  thin -as  to  be  very  transparent,  are  relatively  more 
muscular,  and  are  more  plentifully  supplied  with  valves.  The 
structure  of  the  latter  is  the  same  as  in  the  veins. 

3.  The  Structure  and  Function  of  Lymphatic  Glands.  — 
Lymphatic  glands  occur  at  more  or  less  frequent  intervals 
along  the  course  of  the  lymphatic  vessels.  They  are  of  very 
variable  size,  being  somewhat  rounded  when  small,  and 
when  large  having  more  or  less  the  shape  of  a  bean.     The 

V 

-A.L 


Fig  40.  —  Diagrammatic  Representation  of  a  Lymphatic  Gland  seen  in 
Section.  (After  Sharpey.) 
Cap.  capsule;  7V.  trabecules:  G.S.  glandular  substance;  L.S.  lymph-sinus. 
In  the  alveolus  marked  /  all  the  leucocytes  are  supposed  to  have  been  washed  out; 
in  the  rest  of  the  gland  they  are  shown  in  the  glandular  substance,  but  washed  out 
of  the  lymph-sinuses.  A.L.  afferent  lymphatic;  E.L.  efferent  lymphatic.  The 
arrows  show  the  direction  in  which  the  lymph  enters  and  leaves  the  gland. 

afferent  lymphatic  vessels  enter  the  gland  by  several 
branches  on  its  more  convex  side,  and  emerge  in  diminished 
numbers  as  efferent  vessels  from  the  opposite  side.  Blood- 
vessels enter  and  leave  the  glands  side  by  side  with  the 
efferent  lymphatic  vessels. 


nb  ELEMENTARY   PHYSIOLOGY  less. 

Each  gland  is  covered  externally  by  a  capsule  or  coat  of 
connective  tissue,  with  which  some  unstriated  muscle  fibres 
are  not  infrequently  mixed.  This  capsule  sends  partitions, 
called  trabeculae,  inwards  and  towards  the  centre  of  the 
gland,  which  divide  it  into  compartments  or  alveoli,  the 
compartments  being  very  regularly  arranged  at  the  outer 
portion  or  cortex  of  the  gland  and  irregularly  in  the  more 
central  parts  or  medulla  (see  Fig.  40).  Each  alveolus  is 
filled  with  a  network  of  connective  tissue,  whose  meshes  are 
small  and  closely  set  in  the  central  part  of  the  alveolus, 
wider  or  more  open  where  in  contact  with  the  trabeculae. 
The  central  small  meshed  network  is  known  as  adenoid  tis- 
sue (p.  53),  is  densely  packed  with  lymph-corpuscles  or 
leucocytes  closely  resembling  the  colourless  corpuscles  of 
blood  (p.  126),  and  constitutes  what  is  usually  spoken  of  as 
the  glandular  substance.  The  more  open-meshed  network 
which  surrounds  the  glandular  substance  and  separates  it 
from  the  trabeculae  is  known  as  the  lymph-sinus  or  lymph- 
channel.  The  meshes  of  the  lymph-sinus,  like  those  of  the 
glandular  substance,  are  crowded  with  leucocytes,  but  these 
are  not  very  firmly  fixed  in  this  network,  as  they  are  in  that 
of  the  glandular  substance,  and  may  be  readily  washed  out 
by  shaking  a  thin  slice  of  the  gland  in  water. 

The  lymphatic  vessels  which  bring  lymph  to  the  gland 
open  directly  into  the  channel  of  the  lymph-sinus,  and  those 
vessels  which  gather  up  the  lymph  to  carry  it  away  from  the 
gland  open  out  of  the  lymph-sinuses. 

The  leucocytes  which  crowd  the  glandular  substance 
present  under  the  microscope  appearances  of  cell  division, 
which  leave  no  doubt  that  they  are  undergoing  rapid  and 
probably  large  increase  in  numbers.  But,  since  the  size  of 
each  gland  is  ordinarily  constant,  a  continual  removal  of  the 
newly  formed  leucocytes  must  be  taking  place.     This  view 


in  LYMPHATIC   GLANDS  117 

is  borne  out  by  the  observation  that  leucocytes  are  more 
numerous  in  the  lymph  coming  from  a  gland  than  in  that 
which  flows  to  it.  The  removal  takes  place  by  a  discharge 
of  leucocytes  from  the  glandular  substance  into  the  meshes 
of  the  neighbouring  lymph-sinus,  whence  they  are  then 
washed  away  in  the  current  of  lymph,  which  is  always  slowly 
flowing  through  the  sinuses.  In  this  way  the  lymphatic 
glands  provide  a  constant  supply  of  leucocytes,  which  are 
passed  ultimately  into  the  blood  and  become  those  white  or 
colourless  corpuscles  with  which  we  shall  have  to  deal  in  the 
next  Lesson. 

4.  Causes  which  lead  to  the  Movements  of  Lymph.  — 
Throughout  the  preceding  description  of  the  lymphatic  sys- 
tem we  have  spoken  of  the  lymph  as  flowing  along  a  series 
of  passages,  from  their  origin  in  the  tissues  to  the  point 
where  they  become  connected  with  the  blood-vessels.  The 
cause  of  this  flow  is  not  so  immediately  apparent  as  it  is  in 
the  case  of  the  blood,  for  the  lymphatic  system  possesses  no 
central  pump,  such  as  the  heart,  to  keep  the  lymph  in  mo- 
tion.1 In  the  absence,  then,  of  any  obviously  propulsive 
mechanism,  to  what  may  we  attribute  this  continual  passage 
of  lymph  along  the  lymphatics? 

The  flow  is  in  reality  brought  about  by  several  causes. 
We  may  point  out,  in  the  first  place,  that  the  processes 
of  filtration  and  diffusion,  whose  nature  will  be  considered 
in  the  next  Lesson,  are  at  work  to  determine  the  initial 
exit  of  fluid- from  the  blood-vessels  into  the  lymph-spaces. 
These  processes  must  obviously  tend  to  drive  out  the  lymph 
already  in  those  spaces  into  and  along  the  channels  leading 

1  The  frog  possesses  four  lymph-hearts,  placed  in  two  pairs  at  the  upper 
and  lower  end  of  the  backbone.  Their  structural  arrangement  is  similar  to 
but  simpler  than  that  of  the  blood-heart,  and,  being  rhythmically  contractile, 
they  pump  the  lymph  into  the  venous  system. 


n8  ELEMENTARY   PHYSIOLOGY  less,  in 

from  them.  Further,  as  we  have  seen,  the  blood-pressure 
in  the  large  veins  near  the  heart  is  very  small  and  is  cer- 
tainly much  less  than  it  is  in  the  capillaries ;  and  since  the 
lymphatics  originate  at  the  capillaries  and  discharge  their 
contents  into  the  great  veins,  this  difference  of  pressure  at 
the  two  ends  of  the  lymphatic  system  must  tend  to  cause 
an  onward  flow  of  lymph.  Here  also  the  movements  of 
respiration  play  a  part,  for,  as  will  be  seen  when  dealing 
with  respiration,  the  pressure  in  the  great  veins  is  suddenly 
diminished  at  each  inspiration,  and  lymph  is  thus  sucked 
out  of  the  thoracic  duct,  no  reversal  of  this  action  being 
possible  at  expiration  because  of  the  valves  guarding  the 
end  of  the  duct.  Finally,  one  great  cause  of  lymph-flow 
is  the  contraction  of  the  muscles  throughout  the  body  and 
the  resulting  pressure  upon  the  lymphatics.  As  in  the 
veins  (p.  61),  so  in  the  lymphatic  vessels;  when  any  press- 
ure is  applied  to  their  outside,  the  lymph  is  driven  out  of 
the  squeezed  part,  and  since  the  valves  open  only  towards 
the  junction  of  the  thoracic  duct  with  the  venous  system,  the 
lymph  is  thereby  driven  along  in  the  desired  direction. 


LESSON    IV 

THE    BLOOD    AND    THE    LYMPH 

1.  Microscopic  Examination  of  Blood.  —  In  order  to 
become  properly  acquainted  with  the  characters  of  the 
blood,  it  is  necessary  to  examine  it  with  a  microscope 
magnifying  at  least  three  or  four  hundred  diameters.  Pro- 
vided with  this  instrument,  a  hand  lens,  and  some  glass 
slides  and  coverslips,  the  student  will  be  enabled  to  follow 
the  present  lesson. 

The  most  convenient  mode  of  obtaining  small  quantities 
of  blood  for  examination  is  to  twist  a  piece  of  string,  pretty 
tightly,  round  the  middle  of  the  last  joint  of  the  middle,  or 
ring  finger,  of  the  left  hand.  The  end  of  the  finger  will 
immediately  swell  a  little,  and  become  darker  coloured,  in 
consequence  of  the  obstruction  to  the  return  of  the  blood 
in  the  veins  caused  by  the  ligature.  When  in  this  condi- 
tion, if  the  finger  be  slightly  pricked  with  a  sharp,  clean 
needle  (an  operation  which  causes  hardly  any  pain),  a 
good-sized  drop  of  blood  will  at  once  exude.  Let  it  be 
deposited  on  one  of  the  glass  slides,  and  covered  lightly 
and  gently  with  a  coverslip,  so  as  to  spread  it  out  evenly 
into  a  thin  layer.  Let  a  second  slide  receive  another  drop, 
and,  to  keep  it  from  drying,  let  it  be  put  under  an  inverted 
watch-glass  or  wine-glass,  with  a  bit  of  wet  blotting-paper 
inside.  Let  a  third  drop  be  dealt  with  in  the  same  way,  a 
few  granules  of  common  salt  being  first  added  to  the  drop. 

119 


120  ELEMENTARY   PHYSIOLOGY  less 

To  the  naked  eye  the  layer  of  blood  upon  the  first  slide 
will  appear  of  a  pale  reddish  colour,  and  quite  clear  and 
homogeneous.  But  on  viewing  it  with  even  a  pocket  lens, 
its  apparent  homogeneity  will  disappear,  and  it  will  look 
like  a  mixture  of  excessively  fine  yellowish-red  particles, 
like  sand,  or  dust,  with  a  watery,  almost  colourless,  fluid. 
Immediately  after  the  blood  is  drawn,  the  particles  will 
appear  to  be  scattered  very  evenly  through  the  fluid,  but 
by  degrees  they  aggregate  into  minute  patches,  and  the 
layer  of  blood  becomes  more  or  less  spotty. 

The  "  particles  "  are  what  are  termed  the  corpuscles  of 
the  blood  ;  the  nearly  colourless  fluid  in  which  they  are 
suspended  is  the  plasma. 

The  second  slide  may  now  be  examined.  The  drop  of 
blood  will  be  unaltered  in  form,  and  may  perhaps  seem  to 
have  undergone  no  change.  But  if  the  slide  be  inclined, 
it  will  be  found  that  the  drop  no  longer  flows  ;  and,  in- 
deed, the  slide  may  be  inverted  without  the  disturbance 
of  the  drop,  which  has  become  solidified,  and  may  be  re- 
moved, with  the  point  of  a  penknife,  as  a  gelatinous  mass. 
The  mass  is  quite  soft  and  moist,  so  that  this  setting,  the 
clotting  or  coagulation,  of  a  drop  of  blood  is  something 
very  different  from  its  drying. 

On  the  third  slide,  this  process  of  clotting  will  be  found 
not  to  have  taken  place,  the  blood  remaining  as  fluid  as 
it  was  when  it  left  the  body.  The  salt,  therefore,  has 
prevented  the  coagulation  of  the  blood.  Thus  this  very 
simple  investigation  teaches  that  blood  is  composed  of  a 
nearly  colourless  plasma,  in  which  many  coloured  corpuscles 
are  suspended  ;  that  it  has  a  remarkable  power  of  clotting  ; 
and  that  this  clotting  may  be  prevented  by  artificial  means, 
such  as  the  addition  of  salt. 

If,  instead  of  using  the  hand   lens,  the   drop   of  blood 


iv  THE   CORPUSCLES   OF  THE   BLOOD  121 

on  the  first  slide  be  placed  under  the  microscope,  the 
particles,  or  corpuscles,  of  the  blood  will  be  found  to  be 
bodies  with  very  definite  characters,  and  of  two  kinds, 
called  respectively  the  red  corpuscles  and  the  white  or 
colourless  corpuscles.     The  former  are  much  more  numer- 


Fig.  41.  —  Red  and  White  Corpuscles  of  the  Blood,  Magnified. 

A.  Moderately  magnified.  The  red  corpuscles  are  seen  lying  in  rouleaux;  at  a 
and  a  are  seen  two  white  corpuscles 

B.  Red  corpuscles  much  more  highly  magnified,  seen  in  face;  C.  ditto,  seen  in 
profile;  D.  ditto,  in  rouleaux,  rather  more  highly  magnified;  E.  a  red  corpuscle 
swollen  into  a  sphere  by  imbibition  of  water. 

F.    A  white  corpuscle  magnified  the  same  as  B. 
H.    Red  corpuscles  puckered  or  crenate  all  over. 
/.   Ditto,  at  the  edge  only. 

ous  than  the  latter,  and  have  a  yellowish-red  tinge  ;  when 
one  of  these  corpuscles  is  seen,  under  a  high  power  of  the 
microscope,  lying  by  itself,  it  seems  to  be  hardly  more  than 
faintly  yellow  in  colour,  but  when  several  are  seen  lying 
one    on    the    other,    the    redness    becomes   obvious.      The 


122  ELEMENTARY   PHYSIOLOGY  less. 

white,  somewhat  larger  than  the  red  corpuscles,  are,  as 
their  name  implies,  pale  and  devoid  of  coloration. 

The  corpuscles  differ  also  in  other  and  more  important 
respects. 

2.  The  Red  Corpuscles. — The  red  corpuscles  (Fig.  41) 
are  flattened  circular  discs,  on  an  average  7/x.  to  8/x  (-g-gV^ 
of  an  inch)  in  diameter,  and  having  about  one-fourth  of  that 
thickness.  It  follows  that  rather  more  than  10,000,000  of 
them  will  lie  on  a  space  one  inch  square,  and  that  the  vol- 
ume of  each  corpuscle  does  not  exceed  tto.o"ooit~o  o7o"7  °f  a 
cubic  inch. 

The  broad  faces  of  the  discs  are  not  flat,  but  somewhat 
concave,  as  if  they  were  pushed  in  towards  one  another. 
Hence  the  corpuscle  is  thinner  in  the  middle  than  at  the 
edges,  and  when  viewed  under  the  microscope,  by  trans- 
mitted light,  looks  clear  in  the  middle  and  darker  at  the 
edges,  or  dark  in  the  middle  and  clear  at  the  edges,  accord- 
ing as  it  is  or  is  not  in  focus.  When,  on  the  other  hand, 
the  discs  roll  over  and  present  their  edges  to  the  eye,  they 
look  like  rods.  All  these  varieties  of  appearance  may  be 
made  intelligible  by  taking  a  small,  round,  flat  disc  of  clay 
or  putty  and  squeezing  the  central  part  of  the  two  flat  sides 
between  the  thumb  and  finger,  so  as  to  make  the  centre 
thinner  than  the  edges ;  the  disc  is  now  more  or  less  similar 
in  shape  to  the  red  corpuscles,  and  may  be  turned  into 
various  positions  before  the  eye. 

In  a  drop  of  blood  immediately  after  it  is  drawn,  the 
red  corpuscles  float  about,  and  roll  or  slide  over  each 
other  quite  freely.  After  a  short  time  (the  length  of  which 
varies  in  different  persons,  but  usually  amounts  to  two  or 
three  minutes),  they  seem,  as  it  were,  to  become  sticky, 
and  tend  to  cohere  ;  and  this  tendency  increases  until,  at 
length,  the  great  majority  of  them  become  applied  face  to 


iv  THE    RED   CORPUSCLES  123 

face,  so  as  to  form  long  series,  like  rolls  of  coin.  The  end 
of  one  roll  cohering  with  the  sides  of  another,  a  network  of 
various  degrees  of  closeness  is  produced  (Fig.  41,  A). 

The  corpuscles  remain  thus  coherent  for  a  certain  length 
of  time,  but  eventually  separate  and  float  freely  again.  The 
addition  of  a  little  water,  or  dilute  acids  or  saline  solutions, 
will  at  once  cause  the  rolls  to  break  up. 

It  is  from  this  running  together  of  the  corpuscles  into 
patches  of  network  that  the  change  noted  above  in  the 
appearances  of  the  layer  of  blood,  viewed  with  a  lens, 
arises.  So  long  as  the  corpuscles  are  separate,  the  sandy 
appearance  lasts;  but  when  they  run  together,  the  layer 
appears  patchy  or  spotted. 

The  red  corpuscles,  rarely,  if  ever,  all  run  together  into 
rolls,  some  always  remaining  free  in  the  meshes  of  the  net. 
In  contact  with  air,  or  if  subjected  to  pressure,  many  of  the 
red  corpuscles  become  covered  with  little  knobs,  so  as  to 
look  like  minute  mulberries  —  an  appearance  which  is  due 
to  the  concentrating,  by  evaporation,  of  the  fluid  in  which 
they  are  floating  (Fig.  41,  H,  H). 

The  red  corpuscles  are  very  soft,  flexible,  and  elastic 
bodies,  so  that  they  readily  squeeze  through  apertures  and 
passages  narrower  than  their  own  diameters,  and  immedi- 
ately resume  their  proper  shapes  (Fig.  36,  G,  H).  Exam- 
ined under  even  a  high  power  the  red  corpuscle  presents 
no  very  obvious  structure;  when,  however,  blood  is  frozen 
and  thawed  one  or  more  times,  or  when  it  is  treated  in 
certain  other  ways,  as,  for  instance,  by  the  addition  of  water, 
the  colouring  matter  which  gave  each  corpuscle  its  yellow 
or  yellowish-red  tinge  is  dissolved  out  and  passes  into  the 
surrounding  fluid,  and  all  that  is  left  of  the  corpuscle  is  a 
colourless  framework  appearing  often  under  the  microscope 
as  a  pale,  hardly  visible,  ring.     Each  corpuscle  in  fact  con- 


i24  ELEMENTARY    PHYSIOLOGY  less, 

sistsof  a  sort  of  spongy  colourless  framework,  the  stroma, 
composed  of  the  kind  of  material  known  as  proteid  and  of 
a  peculiar  colouring  matter,  which,  in  the  natural  condition, 
is  intimately  connected  with  this  framework,  but  may  by 
appropriate  means  be  removed  from  it.  This  colouring 
matter,  which  is  of  a  highly  complex  nature,  is  called 
haemoglobin,  and  may  by  proper  chemical  treatment  be 
resolved  into  a  reddish-brown  substance  containing  iron, 
called  hsematin,  and  a  colourless  proteid  substance. 

Each  corpuscle  therefore  is  not  to  be  considered  as  a 
bag  or  sack  with  a  definite  skin  or  envelope  containing 
fluid,  but  rather  as  a  sort  of  spongy  semi-solid  or  semi-fluid 
mass,  like  a  disc  of  soft  jelly ;  and  as  such  is  capable  of 
imbibing  water  and  swelling  up,  or  giving  out  water  and 
shrinking,  according  to  the  density  of  the  fluid  in  which  it 
may  be  placed.  Thus,  if  the  plasma  of  blood  be  made 
denser  by  dissolving  saline  substances,  or  sugar,  in  it,  water 
is  drawn  from  the  substance  of  the  corpuscle  to  the  dense 
plasma,  and  the  corpuscle  becomes  still  more  flattened  and 
very  often  much  wrinkled.  On  the  other  hand,  if  the 
plasma  be  diluted  with  water,  the  latter  forces  itself  into 
and  dilutes  the  substance  of  the  corpuscle,  causing  the 
latter  to  swell  out,  and  even  become  spherical ;  and,  by 
adding  dense  and  weak  solutions  alternately,  the  corpuscles 
may  be  made  to  become  successively  spheroidal  and  dis- 
coidal.  Exposure  to  carbonic  acid  gas  seems  to  cause 
the  corpuscles  to  swell  out ;  oxygen  gas,  on  the  contrary, 
appears  to  flatten  them. 

The  stroma  or  framework  constitutes  but  a  very  small 
part,  10  per  cent.,  of  the  solid  matter  of  which  the  red 
corpuscles  are  composed,  the  remaining  90  per  cent,  con- 
sisting of  the  colouring  matter  or  haemoglobin.  The  cor- 
puscles may,  therefore,  be  regarded  simply  as  so  many  tiny 


IV 


H.EMOGLOBIN   CRYSTALS 


125 


masses  of  haemoglobin.  Now  haemoglobin,  we  may  say  at 
once,  possesses  the  remarkable  property  of  uniting  in  a 
peculiar  way  with  considerable  quantities  of  oxygen,  and 
thus  confers  on  the  red  corpuscles  their  one  great  charac- 
teristic of  acting  as  the  carriers  of  oxygen  from  the  lungs 
to  the  tissues  of  all  parts  of  the  body. 

The  colouring  matter  of  the  corpuscles  is  further  charac- 
terised by  its  property  of  crystallising  more  or  less  readily. 


Fig.  42.  —  Crystals  of  Haemoglobin.     (After  Funke.) 
a,  squirrel;  b,  guinea-pig;  c,  cat  or  dog;  d,  man;  e,  hamster,  a  European  rodent. 

If  a  small  quantity  of  rat's  or  dog's  blood,  from  which  the 
fibrin  has  been  removed  (see  p.  136),  be  shaken  up  with  a 
small  quantity  of  ether,  it  loses  its  opacity  and  becomes 
quite  transparent  in  thin  layers,  or  as  it  is  often  called  "laky." 
The  transparency  results  from  the  discharge  of  the  haemo- 
globin from  the  stroma  into  the  neighbouring  fluid,  in  which 
it  is  now  in  solution.    If  the  vessel  containing  the  laky  blood 


126  ELEMENTARY    PHYSIOLOGY  less. 

be  allowed  to  .stand  on  ice  for  some  hours,  a  sediment  usu- 
ally forms  at  the  bottom,  and  will  be  found  in  a  successful 
experiment,  when  examined  with  the  microscope,  to  consist 
chiefly  of  blood-crystals  (Fig.  42).  The  crystals  differ  in 
shape  according  to  the  animal  from  whose  blood  they  were 
obtained  ;  in  man  they  have  the  shape  of  prisms.  The  haemo- 
globin of  human  blood  crystallises  with  difficulty,  but  that  of 
the  guinea-pig,  rat,  or  dog,  much  more  readily. 

3.  The  White  Corpuscles. — -The  colourless  corpuscles 
(Fig.  41,  a,  a,  F)  are  larger  than  the  red  corpuscles,  their 
average  diameter  being  10/x  ( 2 ^ 0  of  an  inch).  They  are 
further  seen,  at  a  glance,  to  differ  from  the  red  corpuscles 
by  the  irregularity  of  their  form,  and  by  their  greater  sticki- 
ness or  adhesiveness,  shown  by  their  tendency  to  attach 
themselves  to  the  glass  slide,  while  the  red  corpuscles  float 
about  and  tumble  freely  over  one  another. 

A  still  more  remarkable  feature  of  the  colourless  corpuscles 
than  the  irregularity  of  their  form  is  the  unceasing  variation 
of  shape  which  they  exhibit  so  long  as  they  are  alive.  The 
form  of  a  red  corpuscle  is  changed  only  by  influences  from 
without,  such  as  pressure,  or  the  like  ;  that  of  the  colourless 
corpuscle  is  undergoing  constant  alteration,  as  the  result  of 
changes  taking  place  in  its  own  substance.  To  see  these 
changes  well,  a  microscope  with  a  magnifying  power  of  five 
or  six  hundred  diameters  is  requisite,  and  some  arrangement 
for  keeping  the  preparation  gently  warmed  (to  400  C),  since 
heat  makes  the  movements  more  active  ;  and,  even  then, 
they  are  so  gradual  that  the  best  way  to  ascertain  their  exist- 
ence is  to  make  a  drawing  of  a  given  colourless  corpuscle 
at  intervals  of  a  minute  or  two.  This  is  what  has  been  done 
with  the  corpuscle  represented  in  Fig.  43,  in  which  a  repre- 
sents the  form  of  the  corpuscle  when  first  observed  ;  b,  its 
form  one  minute  afterwards  ;  c,  that  at  the  end  of  the  second 


iv  THE   WHITE   CORPUSCLES  12; 

minute  ;  d,  that  at  the  end  of  the  third;  and  e,  that  at  the 
end  of  the  fifth  minute. 

Careful  watching  of  a  colourless  corpuscle,  in  fact,  shows 
that  every  part  of  its  surface  is  constantly  changing  —  under- 
going active  contraction  or  being  passively  dilated  by  the 
contraction  of  other  parts.  It  exhibits  contractility  in  its 
lowest  and  most  primitive  form. 


Fig.  43.  —  Successive  Forms  assumed   by  Colourless  Corpuscles  of  Human 
Blood.     (Magnified  about  600  diameters.) 

The  intervals  between  the  forms  a,  b,  c,  d,  were  one  minute  each;  between  d  and 
t  two  minutes;  so  that  the  whole  series  of  changes  from  a  to  e  took  five  minutes. 

While  they  are  thus  living  and  active,  a  complete  know- 
ledge of  the  structure  of  the  colourless  corpuscles  cannot  be 
arrived  at.  Each  corpuscle  seems  to  consist  simply  of  a 
mass  of  coarsely  or  finely  granular  protoplasm  (p.  32),  in 
which  no  distinction  of  parts  can  be  seen  (Fig.  41,  F}. 
This  is  especially  the  case  when  the  corpuscle  is  at  rest  and 
assumes  a  spheroidal  shape.  Sometimes,  however,  the  cor- 
puscle, in  the  course  of  the  movements  just  described,  spreads 
itself  out  into  a  very  thin  flat  film  ;  and  when  that  is  the 
case  there  may  be  seen  in  its  interior  a  rounded  body,  dif- 
fering in  appearance  from  the  rest  of  the  body  of  the  cor- 
puscle. Again,  when  a  drop  of  blood  is  diluted  with  water, 
still  better  with  very  dilute  acetic  acid,  the  spongy  pro- 
toplasm of  the  white  corpuscles  swells  up  and  becomes 
transparent,  many  of  the  granules  becoming  dissolved,  and 
in  this  case  the  same  rounded  body  becomes  visible.  This 
internal  rounded  body,  which  differs  in  nature  from  the  rest 
of  the  substance  of  the  corpuscles,  is  the  nucleus  \  and  when 


128  ELEMENTARY   PHYSIOLOGY  :  less, 

the  blood  is  treated  under  the  microscope  with  various 
staining  fluids,  such  as  solutions  of  carmine  or  logwood, 
the  nucleus  generally  stains  more  deeply  than  the  rest  of 
the  corpuscle. 

The  colourless  corpuscle,  with  its  nucleus,  is  a  typical 
nucleated  cell  (p.  31).  It  will  be  observed  that  it 
lives  in  a  free  state  in  the  plasma  of  the  blood,  and  that  it 
exhibits  an  independent  contractility.  In  fact,  except  that 
it  is  dependent  for  the  conditions  of  its  existence  upon  the 
plasma,  it  might  be  compared  to  one  of  those  simple  organ- 
isms which  are  met  with  in  stagnant  water,  and  are  called 
Amoeba,  whence  the  name  "  amoeboid  "  given  to  the  move- 
ments of  the  colourless  corpuscles  of  blood. 

While  the  colourless  corpuscles  are  thus  nucleated  cells, 
the  red  corpuscles  have  no  such  nucleus ;  and  this  is  true 
not  only  of  human  blood  but  of  the  blood  of  all  mammals, 
i.e.  of  all  those  animals  which  suckle  their  young;  in  all 
these  the  red  corpuscle  has  no  nucleus.  In  the  case  of 
birds,  reptiles,  and  fishes,  however,  the  red  corpuscles  as 
well  as  the  colourless  are  nucleated ;  and  in  the  em- 
bryos l  even  of  mammals  the  red  corpuscles  are  at  first 
nucleated. 

The  body  of  the  colourless  corpuscle  may  sometimes  be 
quite  clear  and  transparent,  though  it  more  usually  appears 
to  be  granular  from  the  presence  in  it  of  minute  particles 
which,  varying  in  size,  are  spoken  of  as  "  fine  "  or  "  coarse." 
We  may  regard  these  particles  as  simply  imbedded  in  the 
ground-substance  of  which  the  cell-body  is  made  up,  and, 
since  they  are  variable  in  size  and  numbers,  as  not  essential 
to  the  structure  of  the  corpuscle.  What  the  real  structure 
of  the  living,  contractile  ground- substance  or  protoplasm 
may  be  is  still  a  matter  of  conjecture  and  dispute. 

1  An  embryo  is  the:  rudimentary  unborn  young  of  any  creature. 


iv  THE   WHITE  CORPUSCLES  129 

When  the  colourless  corpuscles  are  examined  chemically 
they  are  found  to  consist  chiefly  of  water,  and  only  10-12 
per  cent,  of  solid  matter.  As  in  the  case  of  the  stroma  of 
the  red  corpuscles,  so  here  also  the  solid  part  is  made  up 
largely  of  proteids  or  substances  closely  allied  to  proteids. 
But  frequently  also  some  small  amount  of  fat  is  found  to  be 
present,  as  also  of  a  representative  of  that  class  of  substances 
known  as  carbohydrates  or  starchy  bodies,  called  glycogen, 
which  will  be  dealt  with  later  on  when  treating  of  the  liver. 
(See  p.  242.) 

The  parts  played  by  the  colourless  corpuscles  in  the 
animal  economy  are  probably  varied  and  numerous,  but 
our  knowledge  of  them  is  very  imperfect.  ^Ye  have  seen 
(p.  108)  that  under  special  circumstances  these  corpuscles 
may,  by  means  of  their  amceboid  movements,  migrate  in 
large  numbers  through  the  walls  of  the  blood-vessels  into 
the  tissues,  and  it  is  possible  that  here  they  may  in  some 
way  assist  in  the  removal  of  the  causes  which  are  giving 
rise  to  a  disturbance.  Quite  probably  a  similar  migration 
is  taking  place  on  a  smaller  scale  at  all  times,  for  some  as 
yet  obscure  but  possibly  similar  purpose.  Again,  by  their 
amoeboid  movements  the  colourless  corpuscles  can  flow 
round  small  solid  particles  and  absorb  them  into  their 
cell-body  ;  in  other  words,  they  can  feed  on  substances  in 
the  blood  and  thus  be  continually  busied  in  keeping  this 
fluid  in  a  normal  condition,  more  particularly  when,  as  in 
disease,  the  composition  of  the  blood  is  altered  by  the  in- 
troduction of  foreign  matter  such  as  bacteria,  etc.  More- 
over, it  is  extremely  probable  that  the  colourless  corpuscles 
may  act  on  the  blood  and  on  any  foreign  matter  it  may  at 
times  contain  by  means  other  than  their  amceboid  move- 
ments ;  namely,  chemically,  by  the  discharge  into  the  blood 
of  substances  formed  within  themselves.     Finally,  there  are 


i3o  ELEMENTARY   PHYSIOLOGY  less. 

•easons  for  supposing  that  when  blood  is  shed,  these  cor- 
puscles have  something  to  do  with  starting  that  striking 
change,  to  which  we  have  already  alluded,  known  as  the 
clotting  or  coagulation  of  blood. 

4.  Blood  Platelets.  —  In  addition  to  the  red  and  white 
corpuscles,  a  third  kind  of  rounded,  colourless  particles 
may,  but  with  difficulty,  be  made  out  as  existing  in  blood. 
These  are  known  as  "  blood  platelets."  They  are  extremely 
minute,  not  much  wider  than  the  thickness  of  a  red  cor- 
puscle, and  usually  disappear  as  soon  as  blood  is  removed 
from  the  body.  But  so  little  is  known  about  them  that  we 
must  not  do  more  than  simply  draw  attention  to  their 
existence. 

5.  The  Origin  and  Fate  of  the  Corpuscles.  — The  exact 
number  of  both  red  and  colourless  corpuscles  present  in  the 
blood  varies  a  good  deal  from  time  to  time ;  and  there  is 
reason  to  think  that  both  kinds  of  corpuscles  are  continually 
being  destroyed  or  made  use  of.  But  since,  on  the  whole, 
the  average  number  of  each  kind  of  corpuscle  is  maintained 
during  healthy  life,  it  is  evident  that  new  corpuscles  must 
be  continually  forming  to  take  the  place  of  those  which  have 
disappeared. 

The  colourless  corpuscles  are,  as  already  described  (p. 
117),  chiefly  formed  out  of  leucocytes  which,  originating  in 
the  lymphatic  glands  and  other  similar  structures,  are  then 
passed  along  the  lymphatic  vessels  into  the  blood. 

Our  knowledge  of  the  origin  of  the  red  corpuscles  is 
somewhat  less  definite  ;  there  Is,  however,  no  doubt  that 
in  the  adult  the  chief  seat  of  their  formation  lies  in  that 
marrow  found  in  the  cavities  of  bones,  which,  from  being 
very  plentifully  supplied  with  blood-vessels,  is  known  as 
red  marrow.  It  seems  wholly  probable  that  the  cells  which 
give  rise  to  red  corpuscles  in  the  marrow  are  a  particular 


iv  THE   PHYSICAL   QUALITIES  OF  BLOOD  131 

kind  of  coloured,  nucleated  cell ;  but  the  question  has  not 
as  yet  been  definitely  decided  as  to  how  the  mammalian  red 
corpuscle  comes  to  have  no  nucleus. 

Apart  from  what  is  known  as  to  the  disappearance  of 
white  corpuscles  from  the  blood  by  migration  through  the 
walls  of  the  vessels,  we  cannot  point  with  certainty  to  any 
other  fate  which  befalls  them. 

When  we  deal  with  the  liver,  however,  we  shall  see  that, 
the  fluid  (bile)  which  it  forms  or  "  secretes  "  is  highly  col- 
oured, though  not  red.  Observation  and  experiment  both 
show  that  the  substance  to  which  the  colour  of  bile  is  due  is 
probably  derived  from  that  coloured  product  of  the  decom- 
position of  haemoglobin  known  as  haematin.  If  haemoglo- 
bin is  thus  the  parent  substance  of  the  colouring  matter 
of  the  bile,  then,  since  bile  is  formed  by  the  liver  each  day 
in  large  quantities,  a  correspondingly  large  daily  destruction 
of  red  corpuscles  must  also  be  taking  place. 

6.  The  Physical  Qualities  of  Blood.  — The  proverb  that 
"  blood  is  thicker  than  water  "  is  literally  true,  as  the  blood 
is  not  only  "  thickened"  by  the  corpuscles,  of  which  it  has 
been  calculated  that  no  fewer  than  70,000,000,000  (nearly 
fifty  times,  the  number  of  the  human  population  of  the 
globe)  are  contained  in  a  cubic  inch,  but  is  rendered 
slightly  viscid  by  the  solid  matters  dissolved  in  the  plasma. 
The  blood  is  thus  rendered  heavier  than  water,  its  specific 
gravity  being  about  1.055.  ^n  other  words,  twenty  cubic 
inches  of  blood  have  about  the  same  weight  as  twenty-one 
cubic  inches  of  water. 

The  corpuscles  are  heavier  than  the  plasma,  and  their 
volume  is  usually  somewhat  less  than  that  of  the  plasma. 
Of  colourless  corpuscles  there  are  usually  not  more  than 
three  or  four  for  every  thousand  of  red  corpuscles  ;  but  the 
proportion  varies  very  much,  increasing  shortly  after  food  is 


132  ELEMENTARY   PHYSIOLOGY  less 

taken,  and  diminishing  in  the  intervals  between  meals. 
Average  blood  may  be  regarded  as  consisting  of  two-thirds 
plasma  and  one-third  corpuscles. 

The  blood  is  hot,  its  temperature  being  about  380  C. 
(100.40  F.). 

7.  The  General  Composition  of  Blood.  —  Considered 
chemically,  the  blood  is  a  faintly  alkaline  fluid,  consisting 
of  water,  of  solid  and  of  gaseous  matters. 

The  proportions  of  these  several  constituents  vary  accord- 
ing to  age,  sex,  and  condition,  but  the  following  statement 
holds  good  on  the  average  :  — 

In  every  100  parts  of  the  blood  there  are  79  parts  of 
water  and  2 1  parts  of  dry  solids  ;  in  other  words,  the  water 
and  the  solids  of  the  blood  stand  to  one  another  in  about 
the  same  proportion  as  the  nitrogen  and  the  oxygen  of  the 
air.  Roughly  speaking,  one-quarter  of  the  blood  is  dry, 
solid  matter  ;  three-quarters  water.  Of  the  2 1  parts  of  dry 
solids,  12  (=7)  belong  to  the  corpuscles.  The  remain- 
ing 9  are  about  two-thirds  (6.7  parts  =  -|)  proteids  (sub- 
stances like  white  of  egg,  coagulating  by  heat),  and  one-third 
( =  \  of  the  whole  solid  matter)  a  mixture  of  saline,  fatty, 
and  carbohydrate  matters  and  sundry  products  of  the  waste 
of  the  body,  such  as  urea. 

The  total  quantity  of  gaseous  matter  contained  in  the  blood 
is  equal  to  rather  more  than  half  the  volume  of  the  blood  ;  that 
is  to  say,  100  c.c.  of  blood  will  contain  about  60  c.c.  of  gases. 
These  gaseous  matters  are  carbonic  acid,  oxygen,  and  nitro- 
gen ;  or,  in  other  words,  the  same  gases  as  those  which 
exist  in  the  atmosphere,  but  in  totally  different  proportions ; 
for  whereas  air  contains  nearly  three-fourths  nitrogen,  one- 
fourth  oxygen,  and  a  mere  trace  of  carbonic  acid,  the  aver- 
age composition  of  the  blood  gases  is  about  two-thirds  or 
more  carbonic  acid,  and  one-third  or  less  oxygen,  the  quan- 


:v  THE   GENERAL   COMPOSITION   OF   BLOOD  133 

tity  of  nitrogen  being  exceedingly  small,  only  1-2  c.c.  in 
100  c.c.  of  blood. 

It  is  important  to  observe  that  blood  contains  much  more 
oxygen  gas  than  could  be  held  in  solution  by  pure  water  at 
the  same  temperature  and  pressure.  This  power  of  holding 
oxygen  depends  upon  the  red  corpuscles,  the  oxygen,  thus 
held  by  them  being  readily  given  up  for  purposes  of  oxida- 
tion. The  connection  between  the  oxygen  and  the  red  cor- 
puscles is  of  a  peculiar  nature,  being  a  sort  of  loose  chemical 
combination  with  one  of  their  constituents,  and  that  con- 
stituent is,  as  we  have  said  previously,  the  haemoglobin ;  for 
solutions  of  haemoglobin  behave  towards  oxygen  almost 
exactly  as  blood  does.  Similarly,  the  blood  contains  more 
carbonic  acid  than  could  be  held  in  solution  by  pure 
water  at  the  same  temperature  and  pressure.  But  unlike 
the  oxygen,  the  carbonic  acid  thus  held  by  blood  is  not 
associated  with  the  haemoglobin  of  the  red  corpuscles ;  in 
fact,  it  seems  to  be  chiefly  retained  by  some  constituents  of 
the  plasma. 

The  corpuscles  differ  chemically  from  the  plasma  in  con- 
taining a  large  proportion  of  the  fats  and  phosphates,  all  the 
iron,  and  almost  all  the  potassium,  of  the  blood;  while  the 
plasma,  on  the  other  hand,  contains  by  far  the  greater  part 
of  the  chlorine  and  the  sodium. 

The  blood  of  adults  contains  a  larger  proportion  of  solid 
constituents  than  that  of  children,  and  that  of  men  more 
than  that  of  women  ;  but  the  difference  of  sex  is  hardly  at 
all  exhibited  by  persons  of  flabby,  or  what  is  called  lym- 
phatic, constitution. 

Animal  diet  tends  to  increase  the  quantity  of  the  red  cor- 
puscles ;  a  vegetable  diet  and  abstinence  to  diminish  them. 
Bleeding  exercises  the  same  influence  in  a  still  more  marked 
degree,    the    quantity  of  red   corpuscles   being  diminished 


134  ELEMENTARY   PHYSIOLOGY  less. 

thereby  in  a  much  greater  proportion  than  that  of  the  other 
solid  constituents  of  the  blood. 

8.  The  Proteids  of  Plasma.  —  By  cooling  or  the  addi- 
tion of  certain  neutral  salts  the  clotting  of  blood  is  retarded 
or  even  entirely  prevented.  The  corpuscles  may  now  be 
removed  and  the  plasma  obtained  as  a  clear,  faintly  yellow 
and  slightly  alkaline  liquid  composed  of  about  90  per  cent, 
water  and  10  per  cent,  solids  in  solution.  The  solids 
consist  chiefly  of  that  kind  of  material  which  we  have  so 
frequently  spoken  of  as  proteids.  Since  these  proteids  are 
typical  of  their  class,  and  since  proteids  are  without  doubt 
the  most  important  substances  met  with  in  the  body,  it  will 
be  as  well  to  state  at  once  what  are  the  essential  character- 
istics of  a  proteid. 

Proteids  are,  in  the  first  place,  extremely  complex  sub- 
stances, so  much  so  that  chemists  have  not  as  yet  been  able 
to  determine  their  constitution  or  assign  any  formula  to 
them.  Some  are  soluble  in  water,  others  only  soluble  in 
solutions  of  a  neutral  salt,  such  as  sodium  chloride,  while 
others  are  insoluble  in  either  of  the  preceding  solvents. 
When  heated,  with  but  few  exceptions  they  are  altered  or 
coagulated,  as  in  the  well-known  change  which  the  white  of 
an  egg,  itself  a  typical  proteid,  undergoes  when  boiled. 

In  the  next  place,  proteids  are  composed  of  the  four  ele- 
ments, carbon,  hydrogen,  oxygen,  and  nitrogen,  with  a  small 
amount  of  sulphur  and  frequently  of  phosphorus  ;  of  these 
the  nitrogen  stands  out  as  having  a  supreme  importance. 
All  the  tissues  of  the  body  contain  nitrogen  and  are  continu- 
ally undergoing  a  nitrogenous  waste,  and  the  body  is  quite 
unable  to  make  use  of  nitrogen  for  the  repair  of  this  waste 
unless  it  is  presented  in  the  form  of  a  proteid.  The  general 
percentage  composition  of  proteids  is,  roughly  speaking,  the 
same  for  all  of  them,  and  varies  but  slightly  on  either  side 


THE    PROTEIDS  OF   PLASMA 


1 3b 


of   the    following    numbers:    carbon    53   parts,   oxygen   22, 
hydrogen  7,  nitrogen  16,  and  sulphur  1-2. 

All  proteids  give  the  three  following  reactions,  (i)  When 
boiled  with  nitric  acid  they  turn  yellow,  and  this  yellow 
turns  to  orange  on  the  addition  of  ammonia,  (ii)  Boiled 
with  Million's  reagent  (a  mixture  of  the  nitrates  of  mercury) 
they  give  a  pink  colour,  (iii)  When  mixed  with  caustic 
soda  and  a  small  amount  of  a  solution  of  sulphate  of  copper 
they  give  a  violet  colour.  These  reactions  suffice  for  the 
detection  of  any  proteid  in  solution  or  as  a  solid. 


Fig.  44.  —  Network  of  Filaments  of  Fibrin  left  after  washing  away  the 
Colouring  Matter  from  a  thin,  flat  Clot  of  Blood.     (Ranvier.) 


The  solids  in  the  plasma  of  blood  are  chiefly  proteids  and 
are  three  in  number.  The  first  is  known  as  fibrinogen,  and 
is  precipitated  by  the  addition  to  plasma  of  15  per  cent,  of 
sodium  chloride  (ordinary  salt).  This  result  is  readily  at- 
tained by  adding  to  the  plasma  an  equal  volume  of  a  satu- 
rated solution  of  sodium  chloride,  which  contains  about  30 
per  cent,  of  salt.     The  fibrinogen  separates  out  from  solu- 


136  ELEMENTARY   PHYSIOLOGY  less. 

tion  as  a  fine,  flocculent,  viscid  precipitate.  Fibrinogen  is 
characterised  by  the  fact  that  it  "sets"  or  coagulates  when 
heated  in  solution  to  560  C.  (1320  F.).  The  second  is 
called  serum-globulin  and  is  similarly  precipitated  when  the 
plasma  from  which  the  fibrinogen  has  been  removed  is 
subsequently  saturated  by  the  addition  of  as  much  sodium 
chloride  as  it  will  dissolve.  It  coagulates,  when  heated  in 
solution,  at  a  temperature  much  higher  than  does  fibrino- 
gen, namely  750  C.  (1670  F.).  The  third  is  known  as 
serum-albumin.  It  may,  roughly  speaking,  be  regarded  as 
very  like  that  kind  of  albumin  with  which  every  one  is  famil- 
iar in  the  white  of  an  egg.  It  coagulates  when  heated  to 
840  C.  (1830  F.)  ;  it  differs  from  serum-globulin  and  also 
from  fibrinogen  by  not  being  precipitated  when  its  solution 
is  saturated  with  sodium  chloride. 

9.  The  Clotting  of  Blood. —  If  a  drop  of  blood  be  spread 
out  in  a  thin  layer  on  a  slide  and  kept  from  drying  it  soon 
becomes  solid  and  gelatinous,  as  in  the  second  experiment 
described  on  p.  120.  When  this  solid  is  carefully  washed, 
by  streaming  water  over  it  very  gently,  the  colouring  matter 
is  removed  and  a  coarse  network  of  extremely  delicate 
fibres  or  filaments  remains.     (Fig.  44.) 

These  filaments  are  formed  in  the  blood  and,  traversing 
it  in  all  directions,  uniting  with  one  another  and  binding  the 
corpuscles  together,  are  the  cause  of  the  blood  having  be- 
come a  semi-solid  mass.  The  filaments  are  composed  of  a 
substance  called  fibrin  ;  hence  it  is  this  formation  of  fibrin 
which  is  the  cause  of  the  solidification  or  clotting  of  the 
blood  ;  but  the  phenomena  of  clotting,  which  are  of  very 
great  importance,  cannot  be  properly  understood  until  the 
behaviour  of  the  blood  when  drawn  in  much  larger  quantity 
than  a  drop  has  been  studied. 

When  a  quantity  of  blood   is  drawn   directly  from   the 


iv  THE   CLOTTING   OF   BLOOD  137 

blood-vessels  of  an  animal  into  a  basin,  it  is  at  first  perfectly 
fluid  ;  but  in  a  very  few  minutes  it  becomes,  through  clot- 
ting, a  jelly-like  mass,  so  solid  that  the  basin  may  be  turned 
upside  down  without  any  of  the  blood  being  spilt.  At  first 
the  clot  is  a  uniform  red  jelly,  but  very  soon  drops  of  a 
clear  yellowish  watery-looking  fluid  make  their  appearance 
on  the  surface  of  the  clot,  and  between  it  and  the  sides  of 
the  basin.  These  drops  increase  in  number,  and  run  to- 
gether, and  after  a  while  it  has  become  apparent  that  the 
originally  uniform  jelly  has  separated  into  two  very  different 
constituents  —  the  one  a  clear,  yellowish  liquid  ;  the  other  a 
red,  semi-solid,  slightly  shrunken  mass,  which  lies  in  the 
liquid.  The  liquid  exudes  from  the  coloured  mass  because 
the  latter  shrinks  and  so  squeezes  it  out. 

The  liquid  is  called  the  serum ;  the  semi-solid  mass  the 
clot.  Now  the  clot  obviously  contains  the  corpuscles  of  the 
"blood,  bound  together  by  some  other  substance ;  and  this 
last,  if  a  small  part  of  the  clot  be  examined  microscopically, 
will  be  found  to  be  that  fibrous-looking  matter,  fibrin,  which 
has  been  seen  forming  in  the  drop  of  blood.  Thus  the  clot 
is  made  up  of  the  corpuscles  plus  the  fibrin  of  the  plasma, 
while  the  serum  is  the  plasma  minus  the  fibrinous  elements 
which  it  contained. 

The  corpuscles  of  the  blood  are  slightly  heavier  than  the 
plasma,  and  therefore,  when  the  blood  is  drawn,  they  tend 
to  sink  very  slowly  towards  the  bottom,  but  as  a  rule  clot- 
ting is  complete  before  the  corpuscles  have  had  time  to 
sink  appreciably.  When,  on  the  other  hand,  the  blood 
clots  slowly,  the  corpuscles  have  so  much  time  to  sink 
that  the  upper  stratum  of  plasma  becomes  quite  free  from 
red  corpuscles  before  the  fibrin  forms  in  it ;  and,  conse- 
quently, the  uppermost  layer  of  the  clot  is  nearly  white  ; 
it  then  receives  the  name  of  the  buffy  coat.     This  is  well 


138  ELEMENTARY   PHYSIOLOGY  less. 

seen  in  the  blood  of  the  horse,  which  clots  with  remarkable 
slowness. 

If  the  blood  is  "  whipped  "  with  a  bunch  of  twigs  as  soon 
as  it  is  drawn  from  the  body,  clotting  takes  place  as  before, 
but  in  this  case  the  clot  is  broken  up  as  fast  as  it  is  formed. 
Under  these  circumstances  the  fibrin  collects  upon  the  twigs, 
and  a  red  fluid  is  left  behind,  consisting  of  the  serum  plus 
the  red  corpuscles  and  many  of  the  colourless  ones.  The 
fibrin  adhering  to  the  twigs  may  readily  be  washed  in  a 
stream  of  water,  and  as  thus  obtained  is  a  white,  stringy, 
elastic  and  very  insoluble  substance.  It  gives,  when  tested, 
all  the  reactions  characteristic  of  proteids,  and  is,  in  fact, 
itself  a  proteid,  although  somewhat  impure. 

The  clotting  of  the  blood  is  hastened,  retarded,  or  tempo- 
rarily prevented  by  many  circumstances. 

(a)  Temperature.  —  A  temperature  up  to  or  slightly 
above  400  C.  (1040  F.)  accelerates  the  clotting  of  the 
blood ;  a  low  one  retards  it  very  greatly ;  so  much  so  that 
blood  kept  at  a  temperature  close  to  freezing  point  may 
remain  fluid  for  a  very  long  time  indeed. 

(b)  The  addition  of  neutral  salts  to  the  blood.  —  Many 
salts,  and  more  especially  sulphate  of  sodium  or  magnesium 
and  sodium  chloride  (common  salt),  dissolved  in  the  blood 
in  sufficient  quantity,  prevent  its  clotting  ;  but  clotting  sets 
in  when  water  is  added  so  as  to  dilute  the  saline  mixture. 

(c)  Contact  with  living  or  not  living  matter.  —  Contact 
with  not  living  matter  promotes  the  clotting  of  the  blood. 
Thus,  blood  drawn  into  a  basin  begins  to  clot  first  where  it 
is  in  contact  with  the  sides  of  the  basin ;  and  a  wire  intro- 
duced into  a  living  vein  will  become  coated  with  fibrin, 
although  perfectly  fluid  blood  surrounds  it. 

On  the  other  hand,  direct  contact  with  living  matter 
retards,  or  altogether  prevents,  the  clotting  of  the  blood. 


iv  THE   CLOTTING   OF   BLOOD  139 

Thus,  blood  remains  fluid  for  a  very  long  time  in  a  portion 
of  a  vein  which  is  tied  at  each  end.  The  heart  of  a  turtle 
remains  alive  for  a  lengthened  period  (many  hours  or  even 
days)  after  it  is  extracted  from  the  body  ;  and,  so  long  as  it 
remains  alive,  the  blood  contained  in  it  will  not  clot,  though, 
if  a  portion  of  the  same  blood  be  removed  from  the  heart,  it 
will  clot  in  a  few  minutes.  Blood  taken  from  the  body  of 
the  turtle,  and  kept  from  clotting  by  cold  for  some  time, 
may  be  poured  into  the  separated,  but  still  living,  heart,  and 
then  will  not  clot. 

The  clotting  of  blood  being  thus  due  to  the  appearance 
in  it  of  fibrin,  we  may  now  consider  how  and  why  the  latter 
is  formed  when  blood  is  shed. 

Clotting  is  an  altogether  physico-chemical  process,  depen- 
dent upon  the  properties  of  certain  of  the  constituents  of  the 
plasma. 

A  comparison  of  plasma  and  serum  shows  that  during 
clotting,  i.e.  during  the  formation  of  fibrin,  one  constituent 
of  the  plasma,  namely,  fibrinogen,  disappears,  the  other 
two  proteids,  serum-globulin  and  serum-albumin,  being  left 
to  appear  in  the  serum.  Many  facts  show  beyond  doubt 
that  the  fibrin  is  formed  out  of  the  fibrinogen.  It  was 
on  this  account  that  the  latter  first  received  the  name 
of  fibrinogen,  or  "  fibrin-maker."  But  there  must  also  be 
some  substance  in  blood  after  it  is  shed  which  leads  to 
the  conversion  of  fibrinogen  into  fibrin ;  for  pericardial  and 
other  serous  fluids  contain  fibrinogen,  but  do  not  usually 
clot,  and  purified  solutions  of  fibrinogen  never  clot  spon- 
taneously.    What  is  this  substance? 

If  serum  be  precipitated  with  an  excess  of  strong  alcohol 
and  after  some  weeks  the  precipitate  is  collected  and  ex- 
tracted with  distilled  water,  this  watery  extract  contains  very 
little  solid  matter,  but  is  found  to  be  active  in  causing  the 


140  ELEMENTARY   PHYSIOLOGY  less. 

conversion  of  fibrinogen  into  fibrin.  We  do  not  as  yet 
know  exactly  what  the  substance  is  in  this  extract  which 
brings  about  the  change  of  the  fibrinogen,  but  for  reasons 
into  which  we  cannot  now  enter,  it  is  classed  with  the 
"ferments,"  of  which  we  shall  have  to  speak  when  we 
come  to  consider  digestion.  These  ferments  are  charac- 
terised by  their  power,  even  when  present  in  small  quan- 
tities, of  producing  great  changes  in  other  bodies  without 
themselves  entering  into  the  changes.  Thus,  the  particu- 
lar ferment  of  which  we  are  speaking,  and  which  has  been 
called  "  fibrin  ferment,"  produces  fibrin,  and  yet  does  not 
itself  become  part  of  the  fibrin  so  produced. 

This  ferment  is  apparently  not  present  in  healthy  blood 
as  it  circulates  in  the  living  blood-vessels,  but  makes  its 
appearance  when  the  blood  is  shed.  We  do  not  know 
exactly  from  what  source  it  comes,  but  there  are  reasons 
for  thinking  that  it  arises  from  a  breaking  down  of  the 
white  corpuscles,  or  it  may  be  of  the  blood  platelets. 

Finally,  then,  although  the  process  of  clotting  is  not  yet 
understood  in  full,  we  may  say  that  fibrin  as  such  does  not 
exist  in  the  blood  at  the  moment  of  its  being  shed,  but  makes 
its  appearance  afterwards  on  account  of  the  action  of  fibrin 
ferment  on  fibrinogen.  It  is  possible  that  other  bodies  are 
concerned  in  the  matter. 

10.  The  Quantity  and  Distribution  of  Blood  in  the 
Body.  —  The  total  quantity  of  blood  contained  in  the  body- 
varies  at  different  times,  and  the  ascertainment  of  its  pre- 
cise amount  is  very  difficult.  It  may  probably  be  estimated, 
on  the  average,  at  not  less  than  one-thirteenth  or  about  7.5 
per  cent,  of  the  weight  of  the  body. 

Its  distribution  at  any  moment  may  be  stated  in  round 
numbers  as  follows  :  — 

One-quarter,  in  the  heart,  the  vessels  of  the  lungs,  and 
the  large  blood-vessels. 


iv  THE   FUNCTIONS   OF  THE   BLOOD  141 

One-quarter,  in  the  vessels  of  the  liver. 

One-quarter,  in  the  vessels  of  the  skeletal  muscles. 

One-quarter,  in  the  vessels  of  the  other  organs  of  the 
body. 

11.  The  Functions  of  the  Blood.  —  The  function  of  the 
blood  is  to  supply  nourishment  to,  and  take  away  waste 
matters  from,  all  parts  of  the  body.  All  the  various  tissues 
may  be  said  to  live  on  the  blood.  From  it  they  obtain  all 
the  matters  they  need,  and  to  it  they  return  all  the  waste 
material  for  which  they  have  no  longer  any  use.  It  is  abso- 
lutely essential  to  the  life  of  every  part  of  the  body  that  it 
should  be  in  such  relation  with  a  current  of  blood  that 
matters  can  pass  freely  from  the  blood  to  it,  and  from  it 
to  the  blood,  by  transudation  through  the  walls  of  the  ves- 
sels in  which  the  blood  is  contained.  And  this  vivifying 
influence  depends  upon  the  corpuscles  of  the  blood.  The 
proof  of  these  statements  lies  in  the  following  experiments  : 

If  the  vessels  of  a  limb  of  a  living  animal  be  tied  in  such 
a  manner  as  to  cut  off  the  supply  of  blood  from  the  limb, 
without  affecting  it  in  any  other  way,  all  the  symptoms  of 
death  will  set  in.  The  limb  will  grow  pale  and  cold,  it 
will  lose  its  sensibility,  and  volition  will  no  longer  have 
power  over  it;  it  will  stiffen,  and  eventually  mortify  and 
decompose. 

But,  if  the  ligatures  be  removed  before  the  death  stiffen- 
ing has  become  thoroughly  established,  and  the  blood  be 
allowed  to  flow  into  the  limb,  the  stiffening  speedily  ceases, 
the  temperature  of  the  part  rises,  the  sensibility  of  the  skin 
returns,  the  will  regains  power  over  the  muscles,  and,  in 
short,  the  part  returns  to  its  normal  condition. 

If,  instead  of  simply  allowing  the  blood  of  the  animal 
operated  upon  to  flow  again,  such  blood,  deprived  of  its 
fibrin   by  whipping,  but  containing  its- corpuscles,  be  arti- 


r42  ELEMENTARY   PHYSIOLOGY  less. 

ficially  passed  through  the  vessels,  it  will  be  found  nearly 
as  effectual  a  restorative  as  entire  blood  ;  while,  on  the 
other  hand,  the  serum  (which  is  equivalent  to  whipped 
blood  without  its  corpuscles)   has  no  such  effect. 

It  is  not  necessary  that  the  blood  thus  artificially  injected 
should  be  that  of  the  subject  of  the  experiment.  Men,  or 
dogs,  bled  to  apparent  death,  may  be  at  once  and  effectu- 
ally revived  by  filling  their  veins  with  blood  taken  from 
another  man,  or  dog ;  an  operation  which  is  known  by 
the  name  of  transfusion. 

Nor  is  it  absolutely  necessary  for  the  success  of  this 
operation  that  the  blood  used  in  transfusion  should  belong 
to  an  animal  of  the  same  species.  The  b\)od  of  a  horse 
will  permanently  revive  an  ass,  and,  speaking  generally, 
the  blood  of  one  animal  may  be  replaced  without  injurious 
effects  by  that  of  another  closely-allied  species ;  while  that 
of  a  very  different  animal  will  be  more  or  less  injurious, 
and  may  even  cause  immediate  death. 

12.  Lymph  :  its  Character  and  Composition.  —  Lymph, 
as  previously  explained,  is  the  fluid  which  fills  the  lymphatic 
vessels,  and  at  the  place  where  it  is  first  formed  is  a  mere 
overflow  of  fluid  from  the  blood  through  the  walls  of  the 
capillaries.  This  exudation  of  fluid  may  also  be  accom- 
panied by  a  migration  of  some  of  the  colourless  corpuscles 
of  the  blood.  Hence  it  i.s  at  once  evident  that,  broadly 
speaking,  lymph  may  be  regarded  as  so  much  blood  minus 
its  red  corpuscles. 

Lymph  is  most  easily  and  plentifully  obtained  for  exami- 
nation from  the  thoracic  duct.  As  procured  from  this  vessel 
it  has  the  advantage  of  being  representative  of  an  average 
specimen  of  lymph,  since  it  is  a  mixture  of  fluid  collected 
from  nearly  all  parts  of  the  body.  But  the  precaution  must 
be  taken  to  collect  the  lymph  from  a  fasting  animal  in  order 


tv  LYMPH  143 

to  avoid  the  complication  due  to  admixture  of  the  lymph 
from  the  body  generally  with  certain  special  substances 
which  are  taken  up  by  the  lymphatics  of  the  intestine  after 
a  meal.  After  a  meal,  the  lymph  from  the  alimentary  canal 
differs  strikingly,  in  one  respect,  as  we  shall  see  later  on, 
from  that  which  comes  from  it  in  the  absence  of  food. 
Lymph  taken,  then,  from  the  thoracic  duct  of  a  fasting  ani- 
mal, is  found  to  be  a  transparent,  faintly  yellow  fluid.  When 
examined  under  the  microscope  it  is  seen  to  contain  a  num- 
ber x  of  corpuscles,  the  lymph-corpuscles  or  leucocytes,  very 
similar  to  the  colourless  corpuscles  of  blood,  though  perhaps 
on  the  whole  rather  smaller,  and  like  the  latter  showing 
amoeboid  movements,  especially  if  kept  warm.  These  leuco- 
cytes may  represent  some  of  the  white  blood-corpuscles  which 
migrated  from  the  vessels,  but  by  far  the  larger  number  are 
formed  in  the  lymphatic  glands  (see  p.  116). 

When  examined  chemically  lymph  is  found  to  contain  the 
same  salts  as  are  present  in  plasma  and  in  about  the  same 
amount :  the  total  solids  are,  however,  considerably  less  than 
in  plasma,2  and  this  is  due  to  a  deficiency  of  proteids.  But 
the  proteids  present  in  lymph  are  the  same  in  kind  as  the 
three  already  described  as  found  in  plasma,  viz.,  fibrinogen, 
serum-globulin,  and  serum-albumin.  Hence  lymph  clots 
when  left  to  itself  and  yields  fibrin  identical  with  that  ob- 
tained from  blood,  only  in  smaller  quantities,  so  that  the  clot 
is  less  firm  than  from  blood.  Some  gas  may  also  be  extracted 
from  it,  but  in  the  absence  of  red  corpuscles  the  amount  of 
oxygen  it  yields  is  scarcely  appreciable  ;  the  bulk  of  the  gas 
is  carbonic  acid. 

1  Equal  on  the  average  to  the  number  present  in  blood,  so  that  in  a  drop 
of  lymph  very  few  would  be  seen  and  often  none  at  all.     - 

2  Only  about  5  per  cent,  of  its  weight  as  compared  with  8  to  10  per  cent, 
in  plasma, 


144  ELEMENTARY  PHYSIOLOGY  less 

Average  lymph  is  therefore  very  similar  to  plasma  some- 
what diluted  with  water ;  but  the  dilution  is  not  the  same  in 
lymph  collected  from  different  parts  of  the  body.  Lymph 
differs  also  in  composition  when  collected  from  the  same 
part  at  different  times.  Usually  this  difference  is  slight,  but 
in  the  case  of  one  source  it  is  marked  and  important.  In  a 
fasting  animal  the  lymph  coming  from  the  intestines  is  essen- 
tially the  same  as  average  lymph ;  but  after  food  has  been 
taken,  and  especially  if  the  food  contains  much  fat,  and  food 
always  contains  some  fat,  this  lymph  appears  to  be  quite 
white  or  "  milky."  Owing  to  the"  thinness  of  the  walls  of 
the  lymphatics  the  contents  are  visible  from  their  exterior, 
so  that  the  vessels  also  appear  white  or  milky,  and  hence 
this  particular  set  of  lymphatics  is  known  as  the  lacteals,  and 
the  contents  are  called  chyle.  The  only  difference  between 
chyle  and  the  lymph  ordinarily  present  in  the  lacteals  is  that 
chyle  holds  in  suspension  a  large  amount  of  fat  (from  5  to 
15  per  cent.)  in  a  state  of  extremely  fine  division.  These 
minute  particles  of  fat  reflect  a  great  deal  of  the  light  falling 
upon  them  and  hence  the  fluid  appears  white.  Some  of  the 
fat  in  chyle  exists  in  the  form  of  minute  globules,  similar  to 
those  present  in  milk,  but  the  larger  part  is  so  finely  divided 
that  it  can  only  be  spoken  of  as  "  granules  "  and  in  this  form 
is  known  as  the  molecular  basis  of  chyle. 

13.  The  Mode  of  Formation  of  Lymph. — In  all  which 
we  have  so  far  said  respecting  lymph  we  have  spoken  of  it 
merely  as  an  exudation  of  fluid  from  the  walls  of  the  capil- 
laries. We  may  now  consider  what  are  the  causes  which 
lead  to  the  presence  of  lymph  in  the  lymph-spaces  of  the 
tissues. 

Two  physical  processes  suggest  themselves  at  once  as 
possible  causes  ;  these  are  filtration  and  diffusion.  Filtra- 
tion consists  in  the  passage  of  fluid  and  of  substances  in 


THE   MODE   OF   FORMATION   OF   LYMPH 


'45 


-& 


solution  tlirough  a  porous  membrane  as  the  result  of  a  differ- 
ence of  pressure  on  the  two  sides  of  the  membrane.  Diffu- 
sion, on  the  other  hand,  is,  broadly  speaking,  independent 
of  such  a  difference  of  pressure.  A  simple  experiment 
shows  at  once  the  essential  feature  of  diffusion.  Tie  a  piece 
of  parchment  paper  tightly  over  the  wide  end  of  an  ordinary 
"  thistle  tube  "  as  used  by  chemists.  Then  fill  the  bulb  and 
about  one  inch  of  the  tube  with  a 
strong  (20  per  cent.)  solution  of 
sugar  or  common  table-salt  and  fix 
the  tube  vertically,  as  in  Fig.  45,  in 
a  beaker  of  water,  so  that  the  surface 
of  the  solution  in  the  tube  is  at  the 
same  level  as  that  of  the  water  in 
the  beaker.  In  a  short  time  the 
sugar  or  salt  begins  to  pass  out 
through  the  paper  and  may  be  de- 
tected in  the  water  in  the  beaker. 
At  the  same  time  water  passes 
through  the  paper  in  the  opposite 
direction  into  the  tube  and  in  con- 
siderable quantity,  so  that  the  liquid 
rises  in  the  narrow  part  of  the  tube 
and  may  ultimately  stand  several 
inches  above  the  surface  of  that 
which  is  in  the  beaker. 

Substituting  the  wall  of  the  capillaries  for  the  paper  used 
in  the  preceding  experiment  we  have  the  conditions  neces- 
sary for  a  possibly  diffusive  interchange  between  the  blood 
on  the  one  side  of  that  wall  and  the  fluid  in  the  tissues  on 
the  other.  We  may  say  at  once  that  diffusion  by  itself  will 
not  account  for  the  formation  of  lymph.  In  support  of  this 
statement  it  may  suffice  to  point  out  that  lymph  contains  a 

L 


Fig.  45. — To  Illustrate  a 
Simple  Experiment  on 
Diffusion. 

1 1.  thistle  tube:  p p,  parch- 
ment paper;  s.  sugar  or  salt 
solution;  b.  beaker;  w.  water 
in  beaker. 


t$6  ELEMENTARY   PHYSIOLOGY  less 

considerable  amount  of  proteids,  and  these  are  characteristi- 
cally non-diffusible.1 

On  the  other  hand,  the  blood-pressure  in  the  capillaries, 
though  much  less  than  in  the  arteries,  is  not  inconsiderable, 
and  is  exerted  against  the  walls  of  these  vessels.  Can  we 
then  account  for  the  formation  of  lymph  as  the  result  of  fil- 
tration ?  Here  again  we  may  at  once  say  that  the  passage 
of  fluid  through  the  walls  of  the  capillaries  under  the  influ- 
ence of  pressure  has  a  great  deal  to  do  with  the  formation 
of  lymph.  We  are  justified  in  this  view  by  the  fact  that,  as 
a  general  rule,  increase  of  blood-pressure  in  the  capillaries 
leads  to  an  increased  flow  of  lymph  from  the  parts  they  sup- 
ply. But  we  must  not  conclude,  therefore,  that  the  process 
is  entirely  due  to  filtration.  Experiments  may  be  made  in 
which  while  we  know  that  the  blood-pressure  in  the  capilla- 
ries is  much  greater  than  usual,  no  increased  formation  of 
lymph  takes  place.  Again,  it  is  possible  by  certain  means 
to  obtain  a  greatly  increased  flow  of  lymph  from  parts  in 
whose  capillaries  there  is  no  obvious  increase  of  blood-press- 
ure. Neither  of  these  results  would  hold  good  in  the  case 
of  any  ordinary  filter.  But  in  the  case  of  lymph,  as  a  matter 
of  fact,  it  is  not  an  ordinary  filter  with  which  we  have  to 
deal.  The  wall  of  a  capillary  is  made  of  cells  which  are 
alive  and  are  thus  able  to  change  their  condition  from  time  to 
time.  By  this  means  the  capillary  wall  is,  as  it  were,  the 
master  of  the  current  of  fluid  passing  across  it  under  varying 
filtrational  pressure,  and  can  determine  by  means  at  present 
unknown  to  us  not  only  how  much  fluid  shall  pass,  but  in 
what  relative  proportions  its  several  constituents  shall  make 
their  exit.     When  once  this  idea  is  clearly  grasped  many 

1  Substances  such  as  the  proteids  of  blood,  also  gelatin,  which  will  diffuse 
either  not  at  all  or  only  with  difficulty,  are  known  as  colloids,  in  contradis- 
tinction to  crystalline  substances  or  crystalloids ',  which  diffuse  readily. 


iv  THE   FUNCTIONS   OF  THE   LYMPH  14* 

difficulties  disappear.  We  can  understand  more  easily  why 
the  lymph  differs  in  composition  as  formed  in  various  parts 
of  the  body.  We  see  why  arterial  dilation  is  less  potent  to 
increase  lymph  formation  than  is  venous  obstruction,  for, 
although  they  both  increase  the  blood-pressure  in  the  capil- 
laries, venous  obstruction  is  necessarily  accompanied  by  a 
stagnation  of  blood  which  presumably  alters  the  condition 
of  the  capillary  wall.  We  can  also  now  more  easily  appreci- 
ate many  details  of  inflammation  as  previously  described 
(p.  108). 

The  formation  of  lymph  may  thus  be  regarded  as  the 
result  of  the  passage  of  certain  constituents  of  the  blood- 
plasma  through  the  walls  of  the  capillaries,  the  two  processes 
of  diffusion  and  filtration  probably  s/iaring  in  the  proceeding, 
but  the  passage  being  made  peculiar  by  the  influence  of  the 
living  walls  of  the  vessels  through  which  it  is  taking  place. 

14.  The  Functions  of  the  Lymph.  —  The  lymph  has 
already  been  spoken  of  (p.  no)  as  a  "  middleman  "  between 
the  blood  on  the  one  hand  and  the  tissue  on  the  other.  With 
the  single  exception  of  the  lining  epithelial  membrane  of  the 
blood-vessels,  no  tissue  deals  directly  with  the  blood  itself. 
The  lymph,  before  it  is  gathered  into  vessels  for  return  to 
the  blood,  surrounds  and  bathes  the  living  cells.  All  sup- 
plies of  food  and  of  oxygen  are  conveyed  from  the  blood 
to  the  cells  by  the  lymph,  and  all  waste  matters  in  going  from 
the  cells  to  the  blood  for  ultimate  excretion  are  carried  by 
the  same  medium.  The  presence  of  the  lymph  and  its  inti- 
mate relation  to  the  living  substance  thus  make  effective  the 
vivifying  influence  of  the  blood.  The  former  is  as  essential 
to  life  as  is  the  latter. 


LESSON   V 


RESPIRATION 


1.  The  Gases  of   Arterial   and  Venous  Blood.  —  The 

blood,  the  general  nature  and  properties  of  which  have 
been  described  in  the  preceding  Lesson,  is  the  highly  com- 
plex product,  not  of  any  one  organ  or  constituent  of  the 
body,  but  of  all.  Many  of  its  features  are  doubtless  given 
to  it  by  its  intrinsic  and  proper  structural  elements,  the 
corpuscles  ;  but  the  general  character  of  the  blood  is  also 
profoundly  affected  by  the  circumstance  that  every  other 
part  of  the  body  takes  something  from  the  blood  and  pours 
something  into  it.  The  blood  may  be  compared  to  a  river, 
the  nature  of  the  contents  of  which  is  largely  determined 
by  that  of  the  head  waters,  and  by  that  of  the  animals 
which  swim  in  it ;  but  which  is  also  very  much  affected  by 
the  soil  over  which  it  flows,  by  the  water-weeds  which  cover 
its  banks,  by  affluents  from  distant  regions,  by  irrigation 
works  which  are  supplied  from  it,  and  by  drain-pipes  which 
flow  into  it. 

One  of  the  most  remarkable  and  important  of  the  changes 
effected  in  the  blood  is  that  which  results,  in  most  parts  of 
the  body,  from  its  simply  passing  through  capillaries,  or,  in 
other  words,  through  vessels  the  walls  of  which  are  thin 
enough  to  permit  a  free  exchange  between  the  blood  and 
the  fluids  which  permeate  the  adjacent  tissues  (p.  56). 

148 


less,  v  THE   GASES   OF   BLOOD  149 

Thus,  if  blood  be  taken  from  the  artery  which  supplies 
a  limb,  it  will  be  found  to  have  a  bright  scarlet  colour ; 
while  blood  drawn,  at  the  same  time,  from  the  vein  of  the 
limb,  will  be  of  a  purplish  hue.  And  as  this  contrast  is  met 
with  in  the  contents  of  the  arteries  and  veins  in  general 
(except  the  pulmonary  artery  and  veins),  the  scarlet  blood 
is  commonly  known  as  arterial  and  the  dark  blood  as 
venous. 

This  conversion  of  arterial  into  venous  blood  takes  place 
in  most  parts  of  the  body  while  life  persists.  Thus,  if 
a  limb  be  cut  off  and  scarlet  blood  be  forced  into  its 
arteries  by  a  syringe,  it  will  issue  from  the  veins  as  dark 
blood. 

When  specimens  of  venous  and  of  arterial  blood  are  sub- 
jected to  chemical  examination,  the  differences  presented 
by  their  solid  and  fluid  constituents  are  found  to  be  very 
small  and  inconstant.  But  the  gaseous  contents  of  the 
two  kinds  of  blood  differ  widely  in  the  proportion  which 
the  carbonic  acid  gas  bears  to  the  oxygen  ;  there  being  a 
smaller  quantity  of  oxygen  and  a  greater  quantity  of  car- 
bonic acid,  in  venous  than  in  arterial  blood. 

Every  100  volumes  of  blood  contain  about  60  volumes 
of  gases.  These  may  be  extracted  by  placing  the  blood  in 
a  vessel  connected  with  the  vacuum  of  a  mercurial  pump. 
The  reduction 'of  pressure  on  the  surface  of  the  blood  leads 
to  a  rapid  exit  of  the  gases  into  the  vacuum  ;  they  can  then 
be  collected  and  measured  and  their  respective  volumes 
determined.  The  composition  of  the  blood-gases  is  thus 
found  to  be  the  following  :  — 

Arterial  Blood.  Venous  Blood. 

Oxygen 20  vols 8-1 2  vols. 

Carbonic  acid     ...     40     " 46 

Nitrogen 1-2  " 1-2       " 


ISO  ELEMENTARY   PHYSIOLOGY  less. 

This  difference  in  their  gaseous  contents  is  the  only 
essential  difference  between  venous  and  arterial  blood,  as 
may  be  demonstrated  experimentally.  For,  if  venous  blood 
be  shaken  up  with  oxygen,  or  even  with  air,  it  gains  oxygen, 
loses  carbonic  acid,  and  takes  on  the  colour  and  properties 
of  arterial  blood.  Similarly,  if  arterial  blood  be  treated 
with  carbonic  acid  so  as  to  be  thoroughly  saturated  with 
that  gas,  it  gains  carbonic  acid,  loses  oxygen,  and  acquires 
the  true  properties  of  venous  blood  ;  though,  for  a  reason 
to  be  mentioned  below,  the  change  does  not  take  place 
so  readily  nor  is  it  so  complete  in  this  case  as  in  the 
former.  The  same  result  is  attained,  though  more  slowly, 
if  the  blood,  in  either  case,  be  received  into  a  bladder,  and 
then  placed  in  the  oxygen,  or  carbonic  acid ;  the  thin  moist 
animal  membrane  allowing  the  change  to  be  effected  with 
perfect  ease,  and  offering  no  serious  impediment  to  the 
passage  of  either  gas. 

Venous  blood  is  characterised  not  only  by  the  large 
amount  of  carbonic  acid  which  it  contains,  but  also  by  the 
fact  that  the  red  corpuscles  have  given  up  a  good  deal  of 
their  oxygen  for  the  purposes  of  oxidation,  or,  as  the 
chemists  would  say,  have  become  reduced.  This  is  the 
reason  why  arterial  blood  is  not  so  easily  converted  into 
venous  blood  by  exposure  to  carbonic  acid  as  is  venous 
blood  into  arterial  by  exposure  to  oxygen.  There  is,  in 
the  former  case,  a  want  of  some  oxidisable  substance  to 
carry  off  the  oxygen  from  and  so  to  reduce  the  red  cor- 
puscles. When  such  an  oxidisable  substance  is  added  (as, 
for  instance,  either  ammonium  sulphide  or  Stokes's  re- 
agent1), the  blood  at  once  and  immediately  becomes  com- 
pletely venous. 

1  This  is  made  by  mixing  some  tartaric  acid  with  a  solution  of  ferrous 
sulphate  and  then  adding  ammonia  until  the'  mixture  is  alkaline. 


v  THE   GASES   OF   BLOOD  151 

Practically  we  may  say  that  the  most  important  difference 
between  venous  and  arterial  blood  is  not  so  much  the  rela- 
tive quantities  of  carbonic  acid  as  that  the  red  corpuscles 
of  venous  blood  have  lost  a  good  deal  of  oxygen,  are 
reduced,  and  ready  at  once  to  take  up  any  oxygen  offered 
to  them. 

Similarly,  the  loss  of  oxygen  by  the  red  corpuscles  is  the 
chief  reason  why  the  scarlet  arterial  blood  turns  to  a  more 
purple  or  claret  colour  in  becoming  venous.  It  has  indeed 
been  urged  that  the  red  corpuscles  are  rendered  somewhat 
flatter  by  oxygen  gas,  while  they  are  distended  by  the  action 
of  carbonic  acid  (p.  124).  Under  the  former  circumstances 
they  may,  not  improbably,  reflect  the  light  more  strongly,  so 
as  to  give  a  more  distinct  coloration  to  the  blood;  while, 
under  the  latter,  they  may  reflect  less  light,  and,  in  that  way, 
allow  the  blood  to  appear  darker  and  duller. 

This,  however,  can  only  be  a  small  part  of  the  whole 
matter ;  for  solutions  of  haemoglobin  or  of  blood-crystals 
(see  p.  125),  even  when  perfectly  free  from  actual  blood- 
corpuscles,  change  in  colour  from  scarlet  to  purple,  accord- 
ing as  they  gain  or  lose  oxygen.  It  has  already  been  stated 
(p.  125)  that  oxygen  most  probably  exists  in  the  blood  in 
loose  combination  with  haemoglobin.  And  further,  a  solu- 
tion of  haemoglobin,  when  thus  loosely  combined  with 
oxygen,  has  a  scarlet  colour,  while  a  solution  of  haemo- 
globin deprived  of  oxygen  has  a  purplish  hue.  Hence 
arterial  blood,  in  which  the  haemoglobin  is  richly  provided 
with  oxygen,  is  naturally  scarlet,  while  venous  blood,  which 
not  only  contains  an  excess  of  carbonic  acid,  but  whose 
haemoglobin  also  has  lost  a  great  deal  of  its  oxygen,  is 
purple. 

The  conditions  under  which  the  gases  exist  in  blood  are 
peculiar  and  important  in  connection  with  a  point  we  shall 


152  ELEMENTARY   PHYSIOLOGY  less 

have  to  discuss  later  on,  namely,  how  venous  blood  becomes 
arterial  in  the  lungs  and  how  arterial  blood  becomes  venous 
in  the  tissues.  As  to  the  nitrogen,  we  may  say  at  once  that 
it  is  apparently  in  a  state  of  simple  solution,  as  though  the 
blood  were  so  much  water.  A  very  small  part  of  the  oxy- 
gen is  similarly  simply  dissolved  in  the  blood,  but  practically 
almost  the  whole  of  it  is  in  a  state  of  loose  chemical  combi- 
nation with  the  hemoglobin  of  the  red  corpuscles.  The  facts 
which  prove  this  are  simple  and  conclusive.  When  blood 
is  subjected  to  a  gradually  increasing  vacuum,  the  oxygen 
does  not  come  off  uniformly  and  progressively,  as  the 
vacuum  is  made  greater,  as  it  would  if  it  were  in  mere 
solution ;  on  the  contrary,  it  escapes  with  a  sudden  rush 
after  the  pressure  has  been  considerably  reduced.  In  the 
absence  of  red  corpuscles  plasma  or  serum  absorbs  only  as 
much  oxygen  as  does  an  equal  quantity  of  water,  namely, 
about  one  volume  per  cent. ;  but  blood,  where  the  red  cor- 
puscles are  present,  may  contain  as  much  as  20  volumes  per 
cent,  of  oxygen.  Finally,  solutions  of  haemoglobin  absorb 
oxygen  as  readily  and  largely  as  blood  does. 

The  conditions  under  which  carbonic  acid  exists  in  the 
blood  may  also  be  shown  to  be  those  of  a  loose  chemical 
combination  ;  but  beyond  this  fact  our  knowledge  is  some- 
what incomplete.  It  is  known,  however,  that  the  carbonic 
acid  is  combined  chiefly  in  some  constituents  of  the  plasma 
and  not  with  the  corpuscles,  and  most  authorities  consider 
that  the  larger  part  is  present  in  plasma  united  with  sodium 
in  the  form  of  sodium  bicarbonate,  NaHCO;1. 

2.  The  Nature  and  Essence  of  Respiration. — All  the 
tissues,  as  we  have  seen,  are  continually  using  up  oxygen. 
Their  life,  in  fact,  is  dependent  on  a  continual  succession  of 
oxidations.  Hence  they  are  greedy  of  oxygen,  while  at  the 
same  time  they  are  continually  producing  carbonic  acid  (and 


v  NATURE   AND    ESSENCE   OF   RESPIRATION  153 

other  waste  products).  The  demand  for  oxygen  is  met  by 
a  supply  from  the  red  corpuscles,  and  the  oxygen  they  give 
up  passes  through  the  walls  of  the  capillaries,  across  the 
lymph,  and  so  to  the  cells  of  which  the  tissue  is  composed. 
At  the  same  time  the  carbonic  acid  passes  across  the  lymph 
in  the  opposite  direction,  through  the  capillary  walls  and 
into  the  blood,  by  which  it  is  at  once  whirled  away  into  the 
veins.  The  blood  therefore  leaves  the  tissue  poorer  in  oxy- 
gen and  richer  in  carbonic  acid  than  when  it  came  to  it ; 
and  this  change  is  the  change  from  the  arterial  to  the 
venous  condition.  This  gaseous  interchange  between  the 
blood  and  the  tissues  is  frequently  spoken  of  as  the  respira- 
tion of  the  tissues  or  internal  respiration. 

On  the  other  hand,  if  we  seek  for  the  explanation  of  the 
conversion  of  the  dark  blood  in  the  veins  into  the  scarlet 
blood  of  the  arteries,  we  find,  first,  that  the  blood  remains 
dark  in  the  right  auricle,  the  right  ventricle,  and  the  pul- 
monary artery ;  secondly,  that  it  is  scarlet  not  only  in  the 
aorta,  but  in  the  left  ventricle,  the  left  auricle,  and  the  pul- 
monary veins. 

Obviously,  then,  the  change  from  venous  to  arterial  blood 
takes  place  in  the  capillaries  of  the  lungs,  for  these  are  the 
sole  channels  of  communication  between  the  pulmonary 
arteries  and  the  pulmonary  veins. 

But  what  are  the  physical  conditions  to  which  the  blood 
is  exposed  in  the  pulmonary  capillaries  ? 

These  vessels  are  very  wide,  thin  walled,  and  closely  set, 
so  as  to  form  a  network  with  very  small  meshes,  which  is 
contained  in  the  substance  of  an  extremely  thin  membrane. 
This  membrane  is  in  contact  with  the  air,  so  that  the  blood 
in  each  capillary  of  the  lung  is  separated  from  the  air  by 
only  a  delicate  pellicle  formed  by  its  own  wall  and  the  lung 
membrane.     Hence  an  exchange  very  readily  takes   place 


154  ELEMENTARY   PHYSIOLOGY  less. 

between  the  blood  and  the  air ;  the  latter  gaining  moisture 
and  carbonic  acid,  and  losing  oxygen.1 

This  is  the  essential  step  in  respiration.  That  it  really 
takes  place  may  be  demonstrated  very  readily  by  the  ex- 
periment described  in  the  first  Lesson  (p.  3),  in  which  air 
expired  was  proved  to  differ  from  air  inspired,  by  containing 
more  heat,  more  water,  more  carbonic  acid,  and  less  oxy- 
gen ;  or,  on  the  other  hand,  by  putting  a  ligature  on  the 
windpipe  of  a  living  animal  so  as  to  prevent  air  from  passing 
into,  or  out  of,  the  lungs,  and  then  examining  the  contents 
of  the  heart  and  great  vessels.  The  blood  on  both  sides  of 
the  heart,  and  in  the  pulmonary  veins  and  aorta,  will  then 
be  found  to  be  as  completely  venous  as  in  the  vena?  cavae 
and  pulmonary  artery. 

But  though  the  passage  of  carbonic  acid  (and  hot  watery 
vapour)  out  of  the  blood  and  of  oxygen  into  it  is  the  essence 
of  the  respiratory  process — and  thus  a  membrane  with 
blood  on  one  side,  and  air  on  the  other,  is  all  that  is  abso- 
lutely necessary  to  effect  the  purification  of  the  blood  — 
yet  the  accumulation  of  carbonic  acid  is  so  rapid,  and  the 
need  for  oxygen  so  incessant,  in  all  parts  of  the  human  body, 
that  the  former  could  not  be  cleared  away,  nor  the  latter 
supplied,  with  adequate  rapidity,  without  the  aid  of  exten- 
sive and  complicated  accessory  machinery  —  the  arrange- 
ment and  working  of  which  must  next  be  carefully  studied. 

3.  The  Organs  of  Respiration.  —  The  back  of  the  mouth 
or  pharynx  communicates  by  two  channels  with  the  external 

1  The  student  must  guard  himself  against  the  idea  that  arterial  blood 
contains  no  carbonic  acid,  and  venous  blood  no  oxygen.  In  passing 
through  the  lungs  venous  blood  loses  only  a  part  of  its  carbonic  acid;  and 
arterial  blood,  in  passing  through  the  tissues,  loses  only  a  part  of  its  oxygen. 
In  blood,  however  venous,  there  is  in  health  always  some  oxygen ;  and  in 
even  the  brightest  arterial  blood  there  is  actually  about  twice  as  much  car 
bonic  acid  as  there  is  of  oxygen.     See  the  table  on  p.  149. 


V  THE    ORGANS    OF    RESPIRATION  155 

air  (see  Fig.  46,  g,f,  e).  One  of  these  is  formed  by  the 
nasal  passages,  which  cannot  be  closed  by  any  muscular 
apparatus  of  their  own  ;  the  other  is  presented  by  the  mouth, 
which  can  be  shut  or  opened  at  will. 

Immediately  behind  the  tongue,  at  the  lower  and  front 
part  of  the  pharynx,  is  an  aperture  —  the  glottis  (Fig.  47, 
Gl)  —  capable  of  being  closed  by  a  sort  of  lid  —  the  epi- 
glottis (Fig.  46,  <?)  — or  by  the  shutting  together  of  its  side 
boundaries,  formed  by  the  so-called  vocal  cords.  The 
glottis  opens  into  a  chamber  with  cartilaginous  walls  —  the 
larynx ;  and  leading  from  the  larynx  downwards  along  the 
front  part  of  the  throat,  where  it  may  be  very  readily  felt,  is 
the  trachea,  or  windpipe  (Fig.  46,  c,  Fig.  47,  TV).  The 
trachea  passes  into  the  thorax,  and  there  divides  into  two 
branches,  a  right  and  a  left,  which  are  termed  the  bronchi 
(Fig.  47,  Br).  Each  bronchus  enters  the  lung  of  its  own 
side,  and  then  breaks  up  gradually  into  a  great  number  of 
smaller  branches,  which  divide  and  subdivide  and  are  called 
the  bronchioles  or  bronchial  tubes. 

Each  bronchial  tube  ends  at  length  in  an  elongated  dila- 
tation, about  J5  of  an  inch  in  diameter  on  the  average  and 
known  as  an  hifundibulum  (Fig.  48,  A,  b).  The  wall  of  an 
infundibulum  sends  flattened  projections  into  its  interior  and 
thus  forms  a  series  of  thin  partitions  by  which  its  cavity  is 
divided  up  into  a  large  number  of  little  sacs  or  chambers, 
averaging  T^  of  an  inch  in  diameter.  These  sacs  are  the 
alveoli  or  air-cells. 

The  infundibula  are  bound  together  in  groups  by  con- 
nective tissue  to  form  larger  masses  termed  lobules.  The 
lobules  are  similarly  bound  together  in  groups  to  form  lobes, 
and  the  several  lobes  are  united  to  form  a  lung.  The  blood- 
vessels, nerves,  and  lymphatics  of  each  lung  are  carried  by 
the  connective  tissue  which  binds  the  whole  together. 


I5& 


ELEMENTARY   PHYSIOLOGY 


LESS. 


If  the  trachea  be  handled  through  the  skin,  it  will  be  found 
to  be  firm  and  resisting.  This  is  due  to  a  series  of  cartilagi- 
nous hoops  which  exist  in  the  outer  part  of  the  wall.  They  are 
surrounded  and  united  by  fibrous  connective  tissue.    They  are 


Fig.  46. — A  Section  of  the  Mouth  and  Nose  taken    vertically,  a  little 
to  the  left  of  the  Middle  Line. 

a,  the  vertebral  column;  l>,  the  oesophagus  or  gullet;  c,  the  trachea  or  windpipe; 
d,  the  thyroid  cartilage  of  the  larynx ;  e,  the  epiglottis ;  _/,  the  uvula ;  g;  the  opening  of 
the  left  Eustachian  tube;  //,  the  opening  of  the  left  lachrymal  duct;  i,  the  hyoid  bone; 
k,  the  tongue;  /,  the  hard  palate;  m,  n,  the  base  of  the  skull;  0,  p,  q,  the  superior, 
middle  and  inferior  turbinal  bones.     The  letters  g,f,  e,  are  placed  in  the  pharynx. 

incomplete  behind,  their  ends  being  united  by  unstriated 
muscle,  where  the  trachea  comes  into  contact  with  the  oesopha- 
gus, or  gullet.  The  trachea  is  lined  by  a  mucous  membrane, 
which  consists  of  an  epithelium  of  ciliated  cells  (Fig.  49),  in- 
terspersed with  mucous  cells  ;  these  lie  on  a  distinct  so-called 


THE  ORGANS   OF   RESPIRATION 


157 


basement  membrane  and  below  this  is  a  small  amount  of 
lymphoid  and  elastic  tissue.  Between  the  mucous  membrane 
and  the  outer  layer  which  carries  the  hoops  of  cartilage,  there 
is  a  certain  amount  of  areolar  connective  tissue,  in  which  some 
small  mucous  glands  are  imbedded ;  this  constitutes  the 
submucous  layer.     The  ciliated  cells  are  elongated  columnar 


Fig.  47. — Back  View  of  the  Neck  and  Thorax  of  a  Human  Subject  from 
which  the  Vertebral  Column  and  whole  Posterior  Wall  of  the  Chest 
are  supposed  to  be  removed. 

M.  mouth;  Gl.  glottis;  TV.  trachea;  L.L.  left  lung;  R.L.  right  lung;  Br.  bron- 
chus; P. A.  pulmonary  artery;  P.V.  pulmonary  veins;  Ao.  aorta;  D.  diaphragm; 
H.  heart;  V.C.I,  vena  cava  inferior. 


cells  with  a  large  and  distinct  nucleus.  During  life  the  cilia 
vibrate  incessantly  backwards  and  forwards,  but  work  on  the 
whole  in  such  a  way  as  to  sweep  both  liquid  (mucus)  and 
solid  particles  outwards  or  towards  the  mouth. 

The  walls  of  the  bronchi  and  bronchial  tubes  have  a  struc- 
ture in  general  similar  to  that  of  the  trachea.     But,  as  the 


<58 


ELEMENTARY   PHYSIOLOGY 


tubes  diminish  in  size,  the  cartilages  become  smaller  and 
more  scattered  and  eventually  disappear.  At  the  same 
time  the  muscular  tissue  increases  in  quantity  and  comes 
to  form  a  complete  layer  outside  the  mucous  membrane. 


D 


^Y' 


Fig.  48. 


A.  Two  infundibula  (5),  with  the  ultimate  bronchial  tube  (a)  which  opens  into 
them.     (Magnified  20  diameters.) 

B.  Diagrammatic  view  of  an  air-cell  of  A  seen  in  action:  a,  epithelium;  b,  parti- 
tion between  two  adjacent  cells,  in  the  thickness  of  which  the  capillaries  run ;  c,  fibres 
of  elastic  tissue. 

C.  Portion  of  injected  lung  magnified:  a,  the  capillaries  spread  over  the  walls 
of  two  adjacent  air-cells;  /■>,  small  branches  of  arteries  and  veins. 

D.  Portion  still  more  highly  magnified. 

Thus,  while  the  trachea  and  bronchi  are  kept  permanently 
open  and  pervious  to  air  by  their  cartilages,  the  smaller 
bronchial  tubes  may  perhaps  be  almost  closed  by  the  con- 
traction of  their  muscular  walls.     Eventually  the  muscular 


THE   ORGANS   OF    RESPIRATION 


15". 


tissue  largely  disappears,  and  the  character  of  the  tissue 
between  the  alveoli  is  quite  different  from  that  of  the  walls 
of  the  bronchial  tubes. 

The  very  thin  partitions  (Fig.  48,  B,  b)  which  separate 
these  alveoli  are  supported  by  much  delicate  and  highly 
elastic  tissue,  and  carry  the  wide  and  close-set  capillaries 
into  which  the  ultimate  ramifications  of  the  pulmonary 
artery  pour  its  blood   (Fig.  48,  C,  D).     The  partitions  are 


Fig.  49.  —  Cilivted   Epithelium   Cells  from  the  Trachea  of  the   Rabbit, 

HIGHLY   MAGNIFIED.       (ScHAFER.) 

>«',  m",  ms,  mucus-secreting  cells  lying  between  the  ciliated  cells  and  seen  in  various 
stages  of  mucin  formation. 


covered  with  extremely  thin,  flattened,  non-ciliated  cells, 
which  may  be  easily  seen  in  the  lung  of  a  young  animal,  but 
are  reduced  to  almost  nothing  in  the  lung  of  an  adult  (Fig. 
48,  B,  a).  Thus,  the  blood  contained  in  the  capillaries  is 
exposed  on  both  sides  to  the  air  —  being  separated  from 
the  cavity  of  the  alveolus  on  either  hand  only  by  the  very 
delicate  pellicle  which  forms  the  wall  of  the  capillary  and 
the  lining  epithelium  of  the  alveolus. 


i Go  ELEMENTARY   PHYSIOLOGY  less 

No  conditions  could  be  more  favourable  to  a  ready  ex- 
change between  the  gaseous  contents  of  the  blood  and 
those  of  the  air  in  the  alveoli  than  the  arrangements  which 
obtain  in  the  pulmonary  tissue.  It  will  readily  be  per- 
ceived, however,  that  with  the  continual  pulmonary  circu- 
lation the  pulmonary  air  would  very  speedily  lose  all  its 
oxygen,  and  become  completely  saturated  with  carbonic 
acid,  if  special  provision  were  not  made  for  its  being  inces- 
santly renewed.  The  renewal  is  brought  about  by  the 
working  of  certain  structural  and  mechanical  arrangements 
which  must  now  be  described  in  detail. 

4.  The  Thorax  and  Pleura.  —  The  lungs  (and  heart) 
are  inclosed  in  what  is  practically  an  air-tight  box,  whose 
walls  are  movable.  This  box  is  the  thorax  or  chest.  In 
shape  it  is  conical,  with  the  small  end  turned  upwards,  the 
back  of  the  box  being  formed  by  the  spinal  column,  the 
sides  by  the  ribs,  the  front  by  the  sternum  or  breast-bone, 
the  bottom  by  the  diaphragm,  and  the  top  by  the  root  of 
the  neck  (Fig.  47). 

The  two  lungs  occupy  almost  all  the  cavity  of  this  box 
which  is  not  taken  up  by  the  heart  (Fig.  50).  Each  is  in- 
closed in  its  serous  membrane,  the  pleura,  a  double  bag 
(very  similar  to  the  pericardium,  the  chief  difference  being 
that  the  outer  bag  of  each  pleura  is,  over  the  greater  part  of 
its  extent,  firmly  adherent  to  the  walls  of  the  chest  and  the 
diaphragm,  while  the  outer  bag  of  the  pericardium  is  for  the 
most  part  loose),  the  inner  bag  closely  covering  the  lung 
and  the  outer  forming  a  lining  to  the  cavity  of  the  chest 1 
(Fig.  25, pi).  So  long  as  the  walls  of  the  thorax  are  entire, 
the  cavity  of  each  pleura  is  practically  obliterated,  that  layer 

1  There  is  a  small  amount  of  fluid  between  the  two  surfaces  of  the  pleura, 
to  facilitate  their  rubbing  easily  against  one  another.  This  "  serous"  fluid 
is  in  reality,  as  is  pericardial  fluid,  a  form  of  lymph. 


THE   THORAX   AND    PLEURA 


161 


of  the  pleura  which  covers  the  lung  being  in  close  contact 
with  that  which  lines  the  wall  of  the  chest ;  but,  if  an  open- 
ing be  made  into  the  pleura,  the  lung  at  once  shrinks  to  a 
comparatively  small  size,  and  thus  develops  a  great  cavity 


Fig.  50.  —  Diagram  of  the  Thorax,  showing  the  Position  of  the  Heart  and 

Lungs. 

1-12,  ribs;  11-12,  floating  ribs;  s,  sternum;  r,  rib;  c.c,  costal  cartilages;  c,  clavicle; 
/,  lungs;  a,  apex  of  heart;  peric,  pericardium,  cut  edge. 


between  the  two  layers  of  the  pleura.  If  a  pipe  be  now 
fitted  into  the  bronchus,  and  air  blown  through  it,  the  lung 
is  very  readily  distended  to  its  full  size  ;  but,  on  being  left 
to  itself,  it  collapses,  the  air  being  driven  out   again  with 

M 


[62  ELEMENTARY   PHYSIOLOGY  less. 

some  force.  The  abundant  elastic  tissues  of  the  walls  of 
the  air-cells  are,  in  fact,  so  disposed  as  to  be  greatly 
stretched  when  the  lungs  are  full ;  and  when  the  cause  of 
the  distension  is  removed,  this  elasticity  comes  into  play  and 
drives  the  greater  part  of  the  air  out  again. 

The  lungs  are  kept  distended  in  the  dead  subject,  so  long 
as  the  walls  of  the  chest  are  entire,  by  the  pressure  of  the 
atmosphere  acting  down  the  trachea,  bronchi,  and  bronchi- 
oles upon  the  inner  surfaces  of  the  walls  of  the  alveoli.  For 
though  the  elastic  tissue  is  all  the  while  pulling,  as  it  were, 
at  the  layer  of  pleura  which  covers  the  lung,  and  attempting 
to  separate  it  from  that  which  lines  the  chest,  it  cannot 
produce  such  a  separation  without  developing  a  vacuum 
between  these  two  layers.  To  effect  this,  the  elastic  tissue 
must  pull  with  a  force  greater  than  that  of  the  external  air 
(or  fifteen  pounds  to  the  square  inch),  an  effort  far  beyond 
its  powers,  which  do  not  equal  one-fourth  of  a  pound  on  the 
square  inch.  But  the  moment  a  hole  is  made  in  the  pleura, 
the  air  enters  into  its  cavity,  the  atmospheric  pressure  inside 
the  lung  is  equalised  by  that  outside  it,  and  the  elastic 
tissue,  freed  from  its  opponent,  exerts  its  full  power  on  the 
lung  and  the  latter  collapses. 

5.  The  Movements  of  Respiration.  —  The  hinder  ends 
of  the  ribs  are  attached  to  the  vertebral  column  so  as  to 
be  freely  movable  upon  it.  The  front  ends  of  the  first 
ten  pairs  of  ribs  are  connected  by  the  costal  cartilages  to 
the  sternum,  the  connection  being  therefore  flexible  (Figs. 
50,  51,  52).  When  left  to  themselves,  the  ribs  take  a  posi- 
tion which  is  inclined  obliquely  downwards  and  forwards. 

Two  sets  of  muscles,  called  intercostals,  pass  between 
the  successive  pairs  of  ribs  on  each  side.  The  outer  set, 
called  external  intercostals  (Fig.  52,  A),  run  from  the  rib 
above,  obliquely  downwards  and  forwards,  to  the  rib  below. 


THE   MOVEMENTS   OF   RESPIRATION 


163 


The  other  sel,  internal  intercostals  (Fig.  52,  B),  cross 
these  in  direction,  passing  from  the  rib  above,  downwards 
and  backwards,  to  the  rib  below. 

The  action  of  these  muscles  is  somewhat  puzzling  at  first, 
but  is  readily  understood  if  the  fact  be  borne  in  mind  that 
when  a  muscle  contracts,  it  tends  to  shorten    the   distance 


The  Bony  Walls  of  the  Thorax. 


a,  I,  vertebral  column;  1-12,  ribs;  c,  sternum;  d,  costal  cartilages;  e,  united  car 
tilages  of  lower  true  ribs. 

betiveen  its  tivo  ends.  Let  a  and  b  in  Fig.  53,  A,  be  two 
parallel  bars,  representing  two  consecutive  ribs,  movable  by 
their  ends  upon  the  upright  c,  which  may  be  regarded  as 
the  vertebral  column  at  the  back  of  the  apparatus  ;  then 
a  line  directed  from  x  to  y  will  be  inclined  downwards  and 
forwards,  and  one  from  w  to  z  will  be  directed  downwards 


164 


ELEMENTARY   PHYSIOLOGY 


and  backwards.  Now  it  is  obvious  from  the  figure  that 
the  distance  between  x  and  y  is  shorter  in  B  than  in  A 
and  much  shorter  than  in  C  ;  hence  when  x  y  is  shortened 
the  bars  will  be  pulled  up  from  the  position  C  or  A  to 
or  towards  the  position  B.  Conversely,  the  shortening  of 
w  z  will  tend  to  pull  the  bars  down  from  the  position  B 
or  the  position  A  to  or  towards  the  position  C. 


Fig.  52. —  View  of  Four  Ribs  of  the  Dot;,  with  the  Intercostal  Muscles. 

a,  the  bony  rib;  b,  the  cartilage;  c,  the  junction  of  bone  and  cartilage;  d,  unossi- 
fied,  e,  ossified,  portions  of  the  sternum.  A ,  external  intercostal  muscle;  B,  internal 
intercostal  muscle.  In  the  middle  interspace,  the  external  intercostal  has  been  re- 
moved to  show  the  internal  intercostal  beneath  it. 


If  the  simple  apparatus  just  described  be  made  of  wood, 
hooks  being  placed  at  the  points  x  y,  and  w  z,  and  an 
elastic  band  be  provided  with  eyes  which  can  be  readily 
put  on  to  or  taken  off  these  hooks,  it  will  be  found  that, 
the  band  being  so  short  as  to  be  put  on  the  stretch  when 


THE   MOVEMENTS   OF   RESPIRATION 


165 


hooked  on  to  either  x  y,  or  w  z,  with  the  bars  in  the  hori- 
zontal position,  A,  the  elasticity  of  the  band,  when  hooked 
on  to  x  and  v,  will  bring  them  up  as  shown  in  B ;  while, 
if  hooked  on  to  w  and  z,  it  will  bring  them  down  as  shown 
in  C. 

Substitute  the  contractility  of  the  external  and  internal 
intercostal  muscles  for  the  shortening  of  the  band,  in  vir- 
tue of  its  elasticity,  and  the  model  will  exemplify  the  action 
of  these  muscles;  the  external  intercostals  in  shortening  will 
tend  to  raise,  and  the  internal  intercostals  to  depress,  the 
bony  ribs. 


Fig  53.  —  Diagram  of  Models  illustrating  the  Action  of  the  External  and 
Internal  Intercostal  Muscles. 

B,  inspiratory  elevation;  C,  expiratory  depression. 

Such  a  model,  however,  does  not  accurately  represent 
the  ribs,  with  their  numerous  and  peculiar  curves,  and 
hence,  while  most  physiologists  are  agreed  that  the  exter- 
nal intercostals  raise  the  ribs,  the  action  of  the  internal 
intercostals  is  not  by  any  means  so  certain. 

The  raising  of  the  ribs  which  results  from  the  action  of 
the  external  intercostal  muscles  is  further  assisted  by  the 
contraction  of  certain  other  muscles,  the  scaleni  and  leva- 
tores  costarum.     The    former    are  stretched    between    the 


;66  ELEMENTARY   PHYSIOLOGY  less. 

cervical  vertebrae  and  the  first  two  ribs,  and  serve  to  raise 
and  fix  these  ribs.  The  latter  are  attached  by  their  upper 
ends  to  the  transverse  processes  of  the  last  cervical  and 
first  eleven  dorsal  vertebrae,  and  each  muscle  is  fastened 
by  its  lower  end  to  the  rib  next  below  the  vertebra  from 
which  the  muscle  itself  springs.  These  muscles  must  also 
raise  the  ribs. 

By  means  of  these  several  muscles,  the  ribs  can  be 
raised  from  their  naturally  downward-slanting  position  into 
one  more  nearly  horizontal.  When  this  takes  place,  the 
front  ends  of  the  ribs  must  move  not  only  upwards  but 
forwards,  and  must  therefore  thrust  the  sternum  slightly 
outwards,  or  away  from  the  vertebral  column.  By  this 
movement  the  size  of  the  thorax  is  of  course  increased 
from  back  to  front,  an  increase  which  may  be  easily  felt 
by  placing  one  hand  on  the  back  and  one  on  the  chest 
of  a  person  who  is  breathing.  Again,  when  the  ribs  are 
raised,  each  rib  must  evidently,  by  its  upward  motion, 
tend  to  occupy  the  position  previously  held  by  the  rib 
next  above  it ;  but  the  arched  curve  of  each  rib  increases 
in  size  from  the  first  to  the  seventh  pair  of  ribs,  so  that 
this  upward  movement  makes  a  rib  with  a  larger  arch  take 
the  place  of  one  with  a  smaller  curve.  This  must  clearly 
result  in  an  increase  in  width  of  the  thorax  from  side  to  side, 
an  increase  which  may,  as  before,  be  readily  felt  by  placing 
the  hands  on  the  opposite  sides  of  the  chest. 

The  floor  of  the  thorax  is  formed  by  the  diaphragm,  a  great 
partition  situated  between  the  thorax  and  the  abdomen,  and 
always  concave  to  the  latter  and  convex  to  the  former  (Fig. 
i,  D).  From  its  middle,  which  is  tendinous,  muscular 
fibres  extend  in  a  sheet  downwards  and  outwards  to  the 
ribs,  and  two  especially  strong  masses,  which  are  called  the 
pillars  of  the  diaphragm,  to  the  spinal  column  (Fig.  54). 


THE    MOVEMENTS    OF    RESPIRATION 


165 


vVhen  these  muscular  fibres  contract,  therefore,  they  tend 
to  make  the  diaphragm  flatter,  and  to  increase  the  capacity 
of  the  thorax  at  the  expense  of  that  of  the  abdomen,  by 
pulling  down  the  bottom  of  the  thoracic  box  (Fig.  55,  A), 
or,  in  other  words,  when  the  diaphragm  is  flattened,  the 
size  of  the  thorax  is  increased  from  above  downwards. 


Fig.  54.  —  The  Diaphragm  of  a  Dog,  viewed  from  the  Lower  or  Abdominal 

Side. 

V.C.I,  the  vena  cava  inferior;  O.  the  oesophagus;  Ao.  the  aorta;  the  broad  white 
tendinous  middle  (B.B.B.)  is  easily  distinguished  from  the  radiating  muscular  fibres 
{A. A. A.)  which  pass  down  to  the  ribs  and  into  the  pillars  (C,  D)  in  front  of  the 
vertebrae. 


By  means  then  of  the  movements  of  the  ribs  and  of  the 
diaphragm  the  size  of  the  thorax  may  be  increased  in  all 
its  dimensions.  Let  us  now  consider  what  must  happen  to 
the  lungs  when  the  thorax  becomes  larger.     The  lungs,  as 


168  ELEMENTARY   PHYSIOLOGY  less. 

we  have  said  (p.  162),  are  kept  distended  by  the  pressure 
of  the  atmosphere  acting  down  the  trachea  and  keeping  the 
outer  walls  of  each  lung  firmly  pressed  against  the  inner 
wall  of  the  chest.  This  being  so,  if  the  wall  of  the  thorax 
tends  to  move  away  from  the  wall  of  the  lung,  as  it  must  do 
when  the  thorax  is  enlarged,  then  the  wall  of  the  lung  must 
follow  the  wall  of  the  thorax,  air  rushing  in  through  the 
trachea  to  increase  the  distension  of  the  elastic  lungs  to 
the  required  extent,  and  to  prevent  the  formation  of  any 
vacuum  between  the  two  pleurae.  This  drawing  of  air  into 
the  lungs  constitutes  an  inspiration. 

At  the  end  of  each  inspiration  the  diaphragm  and  the 
external  intercostal  muscles  relax.  The  diaphragm  rises  to 
its  former  position  (Fig.  55,  B),  being  partly  pushed  up  by 
the  abdominal  viscera  which  were  pushed  down  when  the 
diaphragm  contracted.  At  the  same  time  gravity  acting  on 
the  ribs  tends  to  lower  them,  and  this  is  assisted  by  the 
elastic  recoil  of  the  lungs  and  of  the  tissues  of  the  chest 
wall  which  has  been  put  on  the  stretch  during  inspiration, 
and  possibly  also  by  the  contraction  of  the  internal  inter- 
costal muscles.  So  much  of  the  elasticity  of  the  lungs  as 
was  called  into  play  by  the  contraction  of  the  diaphragm 
and  the  raising  of  the  ribs  now  comes  into  action.  By  these 
means  the  thorax  is  diminished  in  size  and  air  is  driven  out 
of  the  lungs,  the  forcing  out  of  the  air  constituting  an 
expiration.  An  expiration  and  an  inspiration  together  con- 
stitute a  respiration. 

Thus  it  appears  that  we  may  have  diaphragmatic  respi- 
ration and  costal  respiration.  As  a  general  rule,  the  two 
forms  of  respiration  coincide  and  aid  one  another,  the  con- 
traction of  the  diaphragm  taking  place  at  the  same  time 
with  that  of  the  external  intercostals,  and  its  relaxation  with 
their  relaxation.     It  is  a  remarkable  circumstance  that  the 


v  THE  MOVEMENTS  OF   RESPIRATION  169 

relative  importance  of  the  two  forms  is  somewhat  different  in 
the  two  sexes.  In  men,  the  diaphragm  takes  the  larger  share 
in  the  process,  the  upper  ribs  moving  comparatively  little ; 
in  women,  the  reverse  is  the  case,  the  respiratory  act  being 
more  largely  the  result  of  the  movement  of  the  ribs. 

In  ordinary  quiet  respiration,  inspiration,  as  has  been  seen, 
is  an  active  process  depending  on  the  contraction  of 
muscles ;  expiration,  on  the  other  hand,  is  rather  due  to  a 
passive  recoil  of  elastic  structures  which  had  been  previously 
put  on  the  stretch.  But  at  times,  as  when  taking  violent 
exercise,  the  respiration  becomes  more  forcible  or,  as  it  is 
called,  "laboured."  In  this  case  many  accessory  muscles 
come  into  play  to  assist  during  inspiration  in  raising  the 
ribs  and  sternum;  being  chiefly  muscles  stretched  between 
the  ribs  and  parts  of  the  vertebral  column  —  above  them  at 
the  back,  and  between  the  neck  and  the  sternum  in  front. 
At  the  same  time  expiration,  from  being  passive,  now  also 
becomes  an  active  process,  chiefly  by  the  contraction  of 
certain  muscles,  the  abdominal  muscles,  which  connect  the 
ribs  and  breast-bone  with  the  pelvis,  and  form  the  front  and 
side  walls  of  the  abdomen.  They  assist  expiration  in  two 
ways  :  first,  directly,  by  pulling  down  the  ribs ;  and  next, 
indirectly,  by  pressing  the  viscera  of  the  abdomen  upwards 
against  the  under  surface  of  the  diaphragm,  and  so  driving 
the  floor  of  the  thorax  upwards. 

It  is  for  this  reason  that,  whenever  a  violent  expiratory 
effort  is  made,  the  walls  of  the  abdomen  are  obviously  flat- 
tened and  driven  towards  the  spine,  the  body  being  at  the 
same  time  bent  forwards. 

In  taking  a  deep  inspiration,  on  the  other  hand,  the  walls 
of  the  abdomen  are  relaxed  and  become  convex,  the  vis- 
cera being  driven  against  them  by  the  descent  of  the  dia- 
phragm —  the  spine  is  straightened,  the  head  thrown  back. 


170  ELEMENTARY   PHYSIOLOGY  less. 

and  the  shoulders  outwards,  so  as  to  afford  the  greatest 
mechanical  advantage  to  all  the  muscles  which  can  elevate 
the  ribs. 

Sighing  is  a  deep  and  prolonged  inspiration  followed  by  a 
long  expiration.  "  Sniffing"  is  a  more  rapid  inspiratory  act, 
in  which  the  mouth  is  kept  shut,  and  the  air  made  to  pass 
through  the  nose. 

A  hiccough  is  the  result  of  a  sudden  inspiration,  due  to 
a  contraction  of  the  diaphragm,  during  which  the  glottis  is 
suddenly  closed  and  the  column  of  air,  striking  on  the 
closed  glottis,  gives  rise  to  the  well-known  and  characteristic 
sound. 

Coughing  is  a  violent  expiratory  act.  A  deep  inspiration 
being  first  taken,  the  glottis  is  closed  and  then  burst'  open 
by  the  violent  compression  of  the  air  contained  in  the  lungs 
by  the  contraction  of  the  expiratory  muscles,  the  diaphragm 
being  relaxed  and  the  air  driven  through  the  mouth.  In 
sneezing,  on  the  contrary,  the  cavity  of  the  mouth  being  shut 
off  from  the  pharynx  by  the  approximation  of  the  soft  palate 
and  the  base  of  the  tongue,  the  air  is  forced  through  the 
nasal  passages. 

It  thus  appears  that  the  thorax,  the  lungs,  and  the  trachea 
constitute  a  sort  of  bellows  without  a  valve,  in  which  the 
thorax  and  the  lungs  represent  the  body  of  the  bellows, 
while  the  trachea  is  the  pipe  ;  and  the  effect  of  the  respira- 
tory movements  is  just  the  same  as  that  of  the  approxima- 
tion and  separation  of  the  handles  of  the  bellows,  which 
drive  out  and  draw  in  the  air  through  the  pipe.  There  is, 
however,  one  difference  between  the  bellows  and  the  respira- 
tory apparatus,  of  great  importance  in  the  theory  of  respi- 
ration, though  frequently  overlooked ;  and  that  is,  that  the 
sides  of  the  bellows  can  be  brought  close  together  so  as  to 
force  out  all,  or  nearly  all,  the  air  which  they  contain ;  while 


THE   MOVEMENTS   OE   RESPIRATION 


i7i 


the  walls  of  the  chest,  when  approximated  as  much  as  possi- 
ble, still  inclose  a  very  considerable  cavity  (Fig.  55,  B)  ;  so 
that,  even  after  the  most  violent  expiratory  effort,  a  very 
large  quantity  of  air  is  left  in  the  lungs. 

If  an  adult  man,  breathing  calmly  in  the  sitting  position,  be 
watched,  the  respiratory  act  will  be  observed  to  be  repeated 


Fig.  55.  —  Diagrammatic  Sections  of  the  Body  in 

A,  inspiration,  B,  expiration.    Tr,  trachea;  St,  sternum:  D,  diaphragm:  Ab,  abdomi- 
nal walls.     The  shading  roughly  indicates  the  stationary  air. 


on  an  average  about  fifteen  to  seventeen  times  every  minute  ; 
but  the  frequency  of  repetition  is  very  variable.  Each  act 
consists  of  certain  components  which  succeed  one  another 
in  a  regular  rhythmical  order.  First,  the  breath  is  drawn 
in  or  inspired ;  immediately  afterwards,  it  is  driven  out  or 


172  ELEMENTARY   PHYSIOLOGY  less. 

expired ;  and  these  successive  acts  are  followed  by  a  brief 
pause.  Thus,  just  as  in  the  rhythm  of  the  heart,  the  auricu- 
lar systole,  the  ventricular  systole,  and  then  a  pause  follow 
in  regular  order ;  so  in  the  chest,  the  inspiration,  the  expira- 
tion, and  then  a  pause  succeed  one  another.  But  in  the 
chest,  unlike  the  case  of  the  heart,  the  pause  is  generally 
very  short  compared  with  the  active  movement ;  indeed, 
sometimes  it  hardly  exists  at  all,  a  new  inspiration  following 
immediately  on  the  close  of  expiration. 

6.  The  Amount  of  Air  Respired.  —  At  each  inspiration 
of  an  adult  well-grown  man  about  500  c.c.  (30  cubic  inches)  of 
air  are  inspired  ;  and  at  each  expiration  the  same,  or  a  slightly 
smaller,  volume  (allowing  for  the  increase  of  temperature  of 
the  air  so  expired)  is  given  out  of  the  body.  To  this  the 
name  of  tidal  air  has  been  conveniently  given. 

The  amount  of  air  which,  as  already  pointed  out,  cannot 
be  got  rid  of  by  even  the  most  violent  expiratory  effort  and 
is  called  residual  air,  is,  on  the  average,  about  1,500  c.c. 
(100  cubic  inches). 

About  as  much  more  in  addition  to  this  remains  in  the 
chest  after  an  ordinary  expiration,  and  is  called  supple- 
mental air. 

Thus  it  follows  that,  after  an  ordinary  inspiration, 
1,500  +  1,500  +  500  =  3,500  c.c.  (100+100  +  30  =  230 
cubic  inches)  may  be  contained  in  the  lungs.  By  taking  the 
deepest  possible  inspiration,  another  1,500  c.c.  (100  cubic 
inches),  called  complemental  air,  may  be  added. 

The  sum  of  the  supplemental,  tidal,  and  complemental  air 
amounts  to  about  3,500  to  4,000  c.c.  (230  to  250  cubic 
inches),  and  is  a  measure  of  what  is  known  as  the  respiratory 
or  vital  capacity.  It  varies  according  to  a  person's  height, 
weight,  and  age. 

It  results  from  these  data  that  the  lungs,  after  an  ordinary 


V  '   THE  AMOUNT  OF  AIR   RESHRED  173 

inspiration,  contain  about  3,500  c.c.  (230  cubic  inches)  of 
air,  and  that  only  about  one-seventh  to  one-eighth  of  this 
amount  is  breathed  out  and  taken  in  again  at  the  next  inspi- 
ration. Apart  from  the  circumstance,  then,  that  the  fresh 
air  inspired  has  to  fill  the  cavities  of  the  hinder  part  of  the 
mouth,  the  trachea,  and  the  bronchi,  if  the  lungs  were 
mere  bags  fixed  to  the  end  of  the  bronchi,  the  inspired  air 
would  descend  so  far  only  as  to  occupy  that  one-fourteenth 
to  one-sixteenth  part  of  each  bag  which  was  nearest  to  the 
bronchi,  whence  it  would  be  driven  out  again  at  the  next 
expiration.  But  as  the  bronchi  branch  out  into  a  prodigious 
number  of  bronchial  tubes,  the  inspired  air  can  only  pene- 
trate for  a  certain  distance  along  these,  and  can  never  reach 
the  air-cells  at  all. 

Thus  the  residual  and  supplemental  air  taken  together 
are,  under  ordinary  circumstances,  stationary  —  that  is  to 
say,  the  air  comprehended  under  these  names  merely  shifts 
its  outer  limit  in  the  bronchial  tubes,  as  the  chest  dilates  and 
contracts,  without  leaving  the  lungs,  and  is  hence  called  sta- 
tionary air;  the  tidal  six,  alone,  being  that  which  leaves  the 
lungs  and  is  renewed  in  ordinary  respiration. 

It  is  obvious,  therefore,  that  the  business  of  respiration  is 
essentially  transacted  by  the  stationary  air,  which  plays  the 
part  of  a  middleman  between  the  two  parties  —  the  blood 
and  the  fresh  tidal  air  —  who  desire  to  exchange  their  com- 
modities :  carbonic  acid  for  oxygen,  and  oxygen  for  carbonic 
acid. 

Now  there  is  nothing  interposed  between  the  fresh  tidal 
air  and  the  stationary  air ;  they  are  gaseous  fluids,  in  com- 
plete contact  and  continuity,  and  hence  the  exchange  be- 
tween them  must  take  place  according  to  the  ordinary  laws 
of  gaseous  diffusion. 

Thus,  the  stationary  air  in  the  air-cells  gives  up  oxygen 


174  ELEMENTARY   PHYSIOLOGY  less. 

to  the  blood,  and  takes  carbonic  acid  from  it,  though  the 
exact  mode  in  which  the  change  is  effected  is  not  thor- 
oughly understood.  By  this  process  it  becomes  loaded  with 
carbonic  acid,  and  deficient  in  oxygen,  though  to  what  pre- 
cise extent  is  not  known.  But  there  must  be  a  very  much 
greater  excess  of  the  one,  and  deficiency  of  the  other,  than  is 
exhibited  by  expired  air,  seeing  that  the  latter  has  acquired 
its  composition  by  diffusion  in  the  short  space  of  time  (four 
or  five  seconds)  during  which  it  has  been  in  contact  with  the 
stationary  air. 

7.  The  Changes  of  Air  in  Respiration.  —  Expired  air 
differs  from  the  air  inspired  in  the  following  particulars  :  — 

(i)  Speaking  generally,  whatever  be  the  temperature  of 
the  external  air,  that  expired  tends  to  be  nearly  as  hot  as 
the  blood,  or  has  a  temperature  of  about  370  C.  (98. 6°  F.). 

(ii)  However  dry  the  external  air  may  be,  that  expired  is 
nearly,  or  quite,  saturated  with  watery  vapour. 

(iii)  While  ordinary  inspired  air  contains  in  100  vol- 
umes — 

Oxygen.  Nitrogen.  Carbonic  Acid. 

20.96  79.OO  .04 

the  composition  of  expired  air  is  on  the  average  in  100 
volumes  — 

Oxygen.  Nitrogen.  Carbonic  Acid 

16.50  79.50  4.00 

Thus,  speaking  roughly,  air  which  has  been  breathed 
once  has  gained  4  per  cent,  of  carbonic  acid  and  lost  rather 
more  than  4  per  cent,  of  oxygen,  the  quantity  of  nitrogen 
being  practically  unchanged. 

(iv)  Expired  air  contains,  in  addition,  small  quantities 
of  "animal  matter"  or  organic  impurities  of  a  highly  de- 
composable kind.  Nothing  is  known  of  their  nature,  but 
they  are  probably  the  chief  cause  why  air  which  has  been 


v  WASTE   WHICH    LEAVES   THE   LUNGS  175 

breathed  once  is  extremely  unwholesome  if  breathed  a 
second  time  ;  hence  they  are  of  great  importance  in  con- 
nection with  ventilation  (see  p.  191). 

(v)  The  volume  of  the  expired  air  is  slightly  (about  J~0) 
less  than  that  of  the  inspired  air.  This  is  due  to  the  fact 
that  the  volume  of  oxygen  which  disappears  is  always 
slightly  greater-  than  the  volume  of  carbonic  acid  which 
takes  its  place ;  for  all  the  oxygen  taken  in  does  not  go  to 
form  carbonic  acid  ;  some  of  it  unites  with  hydrogen  to 
form  water  and  some  with  other  elements  such  as  sulphur. 
Furthermore,  careful  analysis  shows  that  the  nitrogen  in 
expired  air  may  vary  very  slightly  :  sometimes  it  is  a  little 
in  excess  of,  sometimes  slightly  less  than,  that  inspired,  and 
sometimes  it  remains  unaltered. 

8.  The  Amount  of  Waste  which,  leaves  the  Lungs. — 
About  10,000  litres  (from  350  to  400  cubic  feet)  of  air  are 
passed  through  the  lungs  of  an  adult  man  taking  little  or  no 
exercise,  in  the  course  of  twenty-four  hours,  and  are  charged 
with  carbonic  acid,  and  deprived  of  oxygen,  to  the  extent 
of  about  4  per  cent.  This  amounts  to  about  450  litres 
(16  cubic  feet)  of  the  one  gas  taken  in,  and  of  the  other 
given  out.  Thus,  if  a  man  be  shut  up  in  a  close  room  hav- 
ing the  form  of  a  cube  seven  feet  in  the  side,  every  particle 
of  air  in  that  room  will  have  passed  through  his  lungs  in 
twenty-four  hours,  and  a  fifth  of  the  oxygen  it  contained 
will  be  replaced  by  carbonic  acid. 

The  quantity  of  carbon  eliminated  in  the  twenty-four 
hours  is  pretty  nearly  represented  by  a  piece  of  pure  char- 
coal weighing  225  grammes  (eight  ounces). 

The  quantity  of  water  given  off  from  the  lungs  in  the 
twenty-four  hours  varies  very  much,  but  may  be  taken  on 
the  average  as  about  500  c.c.  (one  pint,  or  about  sixteen 
ounces).  It  may  fall  below  this  amount,  or  increase  to 
double  or  treble  the  quantity. 


176  ELEMENTARY   PHYSIOLOGY  less. 

The  air  expired  during  the  first  half  of  an  expiration  con- 
tains less  carbonic  acid  than  that  expired  during  the  sec- 
ond half.  Further,  when  the  frequency  of  respiration  is 
increased  without  altering  the  volume  of  each  inspiration, 
though  the  percentage  of  carbonic  acid  in  each  inspiration 
is  diminished,  it  is  not  diminished  in  the  same  ratio  as  that 
in  which  the  number  of  inspirations  increases;  and  hence 
more  carbonic  acid  is  got  rid  of  in  a  given  time. 

Thus,  if  the  number  of  inspirations  per  minute  is  in- 
creased from  fifteen  to  thirty,  the  percentage  of  carbonic 
acid  evolved  in  each  expiration  in  the  second  case  remains 
more  than  half  of  what  it  was  in  the  first  case,  and  hence 
the  total  evolution  is  greater. 

The  activity  of  the  respiratory  process  is  greatly  modified 
by  the  circumstances  in  which  the  body  is  placed.  Thus, 
cold  greatly  increases  the  quantity  of  air  which  is  breathed, 
the  quantity  of  oxygen  absorbed,  and  of  carbonic  acid  ex- 
pelled ;  exercise  and  the  taking  of  food  have  a  correspond- 
ing effect. 

In  proportion  to  the  weight  of  the  body,  the  activity  of 
the  respiratory  process  is  far  greatest  in  children,  and  dimin- 
ishes gradually  with  age. 

The  excretion  of  carbonic  acid  is  greatest  during  the  day, 
and  gradually  sinks  at  night,  attaining  its  minimum  at  about 
9  p.m.  and  remaining  there  for  six  or  seven  hours. 

Indeed,  it  would  appear  that  the  rule  that  the  quantity  of 
oxygen  taken  in  by  respiration  is,  approximately,  equal  to 
that  given  out  by  expiration,  only  holds  good  for  the  total 
result  of  twenty-four  hours'  respiration.  More  oxygen  ap- 
pears to  be  given  out  during  the  daytime  (in  combination 
with  carbon  as  carbonic  acid)  than  is  absorbed  ;  while,  at 
night,  more  oxygen  is  absorbed  than  is  excreted  as  carbonic 
acid  during  the  same  period.     And  it  is  very  probable  that 


v  CHANGES   IN  THE   LUNGS  AND  TISSUES  177 

the  deficiency  of  oxygen  towards  the  end  of  the  waking 
hours,  which  is  thus  produced,  is  one  cause  of  the  sense  of 
fatigue  which  comes  on  at  that  time.  This  difference  be- 
tween day  and  night  is,  however,  not  constant,  and  appears 
to  depend  a  good  deal  on  the  time  when  food  is  taken. 

The  quantity  of  oxygen  which  disappears  in  proportion 
to  the  carbonic  "acid  given  out,  is  greatest  in  carnivorous, 
least  in  herbivorous  animals  —  greater  in  a  man  living  on  a 
flesh  diet,  than  when  the  same  man  is  feeding  on  vegetable 
matters. 

9.  The  Nature  of  the  Respiratory  Changes  in  the  Lungs 
and  Tissues.  —  When  a  gas  is  inclosed  in  a  vessel,  it  exerts 
a  pressure  on  its  walls.  If  two  gases  are  mixed,  each  gas 
exerts  its  own  pressure  just  as  if  the  other  gas  were  not 
present ;  the  total  pressure  of  the  mixture  is  equal  to  the  sum 
of  the  separate  pressures.  The  pressure  due  to  each  gas  in 
the  mixture  is  called  the  partial  pressure  of  that  gas,  and  is 
proportional  to  the  quantity  of  the  gas.  Hence  if  the  total 
pressure  of  the  mixture  is  measured  and  its  composition  is 
determined  by  analysis,  the  partial  pressure  of  each  gas  is  at 
once  known.  Take,  for  instance,  ordinary  air  when  the 
barometer  stands  at  760  mm.  (30  inches  of  mercury).  The 
partial  pressure  of  the  oxygen  is  -^fa  x  760=  159.6  mm. 
(6.3  inches  of  mercury),  and  that  of  the  nitrogen  is  T7^  x 
760  =  600.4  mm.  (23.7  inches  of  mercury). 

When  a  gas  is  in  contact  with  a  liquid  some  of  the  gas  is 
absorbed  by  the  liquid,  the  amount  being  dependent  on  the 
pressure  of  the  gas.  If  two  gases  are  in  contact  with  the 
same  liquid,  they  will  be  absorbed  in  quantities  proportional 
to  their  respective  partial  pressures  in  the  space  over  the 
liquid,  and  when  the  absorption  is  complete  the  partial  press- 
ures of  the  gases  in  the  liquid  are  the  same  as  the  partial 
pressures  of  the  gases  in  the  space.     If  the  partial  pressure 

N 


78  ELEMENTARY   PHYSIOLOGY  less. 

of  one  of  the  gases  be  made  less  in  the  space  over  the  liquid, 
then  some  of  that  gas  will  make  its  exit  from  the  liquid  ;  and 
if  its  partial  pressure  be,  on  the  other  hand,  increased,  then 
more  of  that  gas  will  enter  the  liquid.  Thus  we  see  that 
changes  in  the  partial  pressures  of  the  gases  in  contact  with 
the  liquid  determine  the  exit  and  entry  of  those  gases  from 
and  into  the  liquid.  Further,  since  gases  diffuse  readily 
through  thin  porous  films,  the  statements  we  have  just  made 
will,  broadly  speaking,  hold  equally  good  in  the  case  when 
the  surface  of  the  fluid  is  separated  from  the  neighbouring 
gases  by  a  thin,  moist,  porous  film.  In  these  facts  we  find 
the  causes  of  the  conversion  of  venous  to  arterial  blood  in 
the  lungs  and  the  reverse  change  in  the  tissues. 

The  air  in  the  alveoli  of  the  lungs  is  a  mixture  of  gases 
separated  from  the  venous  blood  by  the  thin,  moist,  filmy 
wall  of  the  alveoli  and  capillaries.  The  partial  pressures  of 
the  gases  of  the  blood  are  known.  The  composition  of 
alveolar  air  has  not  been  determined  as  yet  because  it  has 
not  been  found  possible  to  collect  air  direct  from  the  alveoli. 
But  from  the  composition  of  expired  air  we  can  at  once 
determine  the  partial  pressures  of  the  oxygen  and  carbonic 
acid  in  it,  and  although  the  partial  pressure  of  the  oxygen 
in  alveolar  air  must  be  less  and  of  carbonic  acid  greater  than 
in  expired  air,  there  are  reasons  for  supposing  that  the  dif- 
ference is  not  great.  By  applying  the  data  thus  obtained  we 
find  that  venous  blood  in  contact  with  oxygen  at  the  partial 
pressure  it  probably  has  in  alveolar  air  readily  takes  up  oxy- 
gen and  becomes  arterialised.  The  entry  of  the  oxygen  is 
further  assisted  by  the  fact  that  the  gas  passes  into  loose 
chemical  combination  in  the  red  corpuscles.  Similarly,  we 
may  say  that  the  exit  of  carbonic  acid  is  due  to  the  differ- 
ence between  the  (lower)  partial  pressure  of  carbonic  acid 
in  the  alveolar  air  and  the  (higher)  partial  pressure  it  has  in 


v         THE   NERVOUS   MECHANISM    OF   RESPIRATION      17$ 

the  venous  blood  ;  but  the  case  is  not  quite  so  clear  as  it  is 
with  regard  to  oxygen,  for  the  partial  pressure  of  carbonic 
acid  in  alveolar  air  is  not  inconsiderable,  and  its  exit  from 
the  blood  is  opposed  by  the  fact  that  it  is  in  loose  combina- 
tion with  some  constituent  of  the  plasma. 

The  blood  thus  fully  arterialised  is  whirled  away  to  the 
tissues.  Here  the  causes  of  the  change  are  much  more 
easily  understood,  for  the  living  tissues  are  greedy  of  oxygen, 
which  they  stow  away  in  compounds  so  stable  that  they 
give  up  no  oxygen  to  the  vacuum  of  even  the  most  powerful 
pump ;  the  partial  pressure  of  oxygen  in  the  tissues  is  in 
fact  zero.  Hence  oxygen  readily  passes  over  from  the 
arterial  blood.  On  the  other  hand,  the  living  tissues  are 
always  producing  carbonic  acid  in  greater  or  less  amount 
according  as  they  are  more  or  less  active  ;  the  partial  press- 
ure of  carbonic  acid  here  is  therefore  high  and  quite  suffi- 
cient to  account  for  the  passage  of  this  gas  from  the  tissues 
into  the  neighbouring  arterial  blood.  The  blood  therefore 
becomes  venous.  The  amount  of  oxygen  left  in  the  blood 
is  dependent  on  the  varying  activity  of  the  tissues,  and  this 
is  the  reason  why  the  volume  of  this  gas  was  given  (p.  149) 
as  varying  from  eight  to  twelve  volumes  in  each  hundred 
volumes  of  venous  blood. 

10.  The  Nervous  Mechanism  of  Respiration.  —  Of  the 
various  mechanical  aids  to  the  respiratory  process,  the  nature 
and  workings  of  which  have  now  been  described,  one,  the 
elasticity  of  the  lungs,  is  of  the  nature  of  a  dead,  constant 
force.  The  action  of  the  rest  of  the  apparatus  is  under 
the  control  of  the  nervous  system,  and  varies  from  time 
to  time. 

As  the  nasal  passages  cannot  be  closed  by  their  own 
action,  air  has  always  free  access  to  the  pharynx  ;  but  the 
glottis,  or  entrance  to  the  windpipe,  is  completely  under  the 


180  ELEMENTARY   PHYSIOLOGY  less. 

control  of  the  nervous  system  —  the  smallest  irritation  about 
the  mucous  membrane  in  its  neighbourhood  being  conveyed, 
by  its  nerves,  to  that  part  of  the  cerebro-spinal  axis  which  is 
called  the  spinal  bulb  or  medulla  oblongata  (see  Lesson 
XII.).  The  spinal  bulb  thus  stimulated  gives  rise,  by  a 
process  which  will  be  explained  hereafter,  termed  reflex 
action,  to  the  contraction  of  the  muscles  which  close  the 
glottis,  and  commonly,  at  the  same  time,  to  a  violent  con- 
traction of  the  expiratory  muscles,  producing  a  cough  (see 
p.  170).  The  muscular  fibres  of  the  smaller  bronchial 
tubes  are  similarly  under  the  control  of  the  bulb,  sometimes 
contracting  so  as  to  narrow  and  sometimes  relaxing  so  as  to 
permit  the  widening  of  the  bronchial  passages. 

These,  however,  are  mere  incidental  actions.  The  whole 
respiratory  machinery  is  worked  by  a  nervous  apparatus. 
From  what  has  been  said,  it  is  obvious  that  there  are  many 
analogies  between  the  circulatory  and  the  respiratory  appa- 
ratus. Each  consists,  essentially,  of  a  kind  of  pump  which 
distributes  a  fluid  (liquid  in  the  one  case,  gaseous  in  the 
other)  through  a  series  of  ramified  distributing  tubes  to  a 
system  of  cavities  (capillaries  or  air-cells),  the  volume  of 
the  contents  of  which  is  greater  than  that  of  the  tubes. 
While  the  heart,  however,  is  a  force-pump,  the  respiratory 
machinery  represents  a  suction-pump. 

In  each  the  pump  is  the  cause  of  the  motion  of  the  fluid, 
though  that  motion  may  be  regulated,  locally,  by  the  con- 
traction or  relaxation  of  the  muscular  fibres  contained  in 
the  walls  of  the  distributing  tubes.  But,  while  the  rhythmic 
movement  of  the  heart  chiefly  depends  upon  an  apparatus 
placed  within  itself,  which  is  then  controlled  by  the  central 
nervous  system,  that  of  the  respiratory  apparatus  results 
mainly  from  the  operation  of  a  nervous  centre  lodged  in  the 
spinal  bulb,  which  has  been  called  the  respiratory  centre. 


v         THE  NERVOUS   MECHANISM   OF   RESPIRATION      181 

This  centre  is  situated  (see  Fig.  56,  R.  C.)  close  to  the 
two  previously  described  as  the  vaso-  motor  and  cardio- 
inhibitory  centres  (Figs.  33  and  34,  pp.  97  and  102).  Im- 
pulses arise  in  this  centre,  pass  down  the  spinal  cord,  and 
leaving  the  cord  along  certain  nerves,  reach  the  various 
muscles  by  whose  contractions  the  movements  of  respira- 
tion are  produced.  The  respiratory  muscles  contract  only 
when  they  receive  these  impulses,  and  therefore  all  the 
movements  of  respiration  depend  upon  the  activity  of  this 
centre,  and  cease  at  once  on  injury  of  this  part  of  the  spinal 
bulb. 

The  action  of  the  centre  is  primarily  automatic ;  in  other 
words,  the  impulses  it  sends  out  appear  to  be  the  result  of 
changes  started  within  itself,  in  the  same  way  that  the  beat 
of  the  heart  is  automatic  as  the  outcome  of  changes  started 
in  the  muscle-tissue  of  which  it  is  made  up.  This  primary 
automatism  of  the  respiratory  centre  is  subject,  however,  to 
control,  in  a  way  to  be  described  presently,  by  impulses 
reaching  it  from  outlying  parts  of  the  body,  and  more  par- 
ticularly by  changes  in  the  condition  or  quality  of  the  blood 
which  circulates  in  the  capillaries  of  the  centre  itself. 

The  intercostal  muscles  are  supplied  by  intercostal  nerves 
coming  from  the  spinal  cord  in  the  region  of  the  back  (Fig. 
56,  I.C.N.),  and  the  muscular  fibres  of  the  diaphragm  are 
supplied  by  two  nerves,  one  on  each  side,  called  the  phrenic 
nerves  (Fig.  56,  Phr.),  which,  starting  from  certain  of  the 
spinal  nerves  in  the  neck,  dip  into  the  thorax  at  the  root  of 
the  neck,  and  find  their  way  through  the  thorax  by  the  side 
of  the  lungs  to  the  diaphragm,  over  which  they  are  dis- 
tributed. From  the  respiratory  centre  in  the  spinal  bulb 
impulses  at  repeated  intervals  descend  along  the  upper  part 
of  the  spinal  cord,  and,  passing  out  by  the  phrenic  and  in- 
tercostal nerves  respectively,  reach  the  diaphragm  and  the 


1S2  ELEMENTARY    PHYSIOLOGY  less. 

intercostal  muscles.  These  immediately  contract,  and  thus 
an  inspiration  takes  place.  Thereupon  the  impulses  cease, 
and  are  replaced  by  other  impulses,  which,  though  starting 
from  the  same  centre,  pass,  not  to  the  diaphragm  and  exter- 
nal intercostal  muscles,  but  to  other,  expiratory,  muscles, 
which  they  throw  into  contraction,  and  thus  expiration  is 
brought  out.  As  a  general  rule,  the  inspiratory  impulses 
are  much  stronger  than  the  expiratory ;  indeed,  in  ordinary 
quiet  breathing  expiration  is  chiefly  brought  about,  as  we 
have  seen,  by  the  elastic  recoil  of  the  lungs  and  chest  walls ; 
these  need  no  nervous  impulses  to  set  them  at  work ;  as 
soon  as  the  inspiratory  impulses  cease  and  the  diaphragm 
and  other  inspiratory  muscles  leave  off  contracting,  they 
come  of  themselves  into  action.  But,  in  laboured  breath- 
ing, very  powerful  expiratory  impulses  may  leave  the  res- 
piratory centre  and  pass  to  the  various  muscles  whose 
contractions  help  to  drive  the  air  out  of  the  chest. 

Everyday  experience  shows  that  no  function  of  the  body 
is  more  obviously  subject  to  sudden  and  marked  changes 
than  is  the  respiration.  It  is  quickened  by  exercise,  quick- 
ened or  slowed  by  emotions;  hurried  by  stimulation  of  the 
skin,  as  by  a  dash  of  cold  water,  or  brought  to  a  standstill  by 
stimulating  the  mucous  membrane  of  the  nose  by  a  pungent 
vapour  such  as  strong  ammonia.  The  changes  involved  in 
sneezing,  laughing,  coughing,  etc.,  are  profound  and  pecul- 
iar. Finally,  we  can  control  our  respiration  by  an  effort  of 
the  will  within  very  wide  limits  and  in  almost  any  desired 
way.  The  mechanism  involved  in  the  production  of  all 
these  changes  is  correspondingly  complicated ;  but  certain 
broad  farts  are  fairly  simple,  and  to  these  we  may  now  turn. 

The  main  trunk  of  the  vagus  nerve,  which,  as  we  shall 
see,  contains  nerve  fibres  coming  from ''the  lungs  (p.  538), 
gives  off  a  branch  to  the  larynx  as  it  passes  down  the  neck 


v         THE   NERVOUS    MECHANISM    OF    RESPIRATION      18} 

(Fig.  56,  S.Lr.).  If  the  vagus  be  cut  below  the poini  of  exit 
of  this  nerve  (as  at  x,  Fig.  56),  and  the  upper  (central)  end 
(y,  Fig.  56)  connected  with  the  spinal  bulb  and  containing 

i  ,'a.f. 


•R.C. 


I.C.N. 


—  Sp.C. 


Fig.  56. — Diagram  to  illustrate  the  Position  of  the  Respiratory  Centre, 
the  Connections  of  this  Centre  with  the  Intercostal  Muscles  and 
Diaphragm,  and  the  Paths  by  which  Impulses  pass  to  the  Centre  from 
Outlying  Parts  of  the  Body  and  from  the  Brain. 

Sp.  C  spinal  cord;  R.C  respiratory  centre  in  the  bulb:  I.C.N,  three  intercostal 
nerves:  Phr.  one  phrenic  nerve  passing  to  the  diaphragm  D. ;  Vg.  vagus  nerve;  V.G. 
ganglion  of  vagus  nerve;  S.Lr.  superior  laryngeal  nerve.  The  dotted  lines,  c.f.,  indi- 
cate paths  of  conduction  for  impulses  to  the  respiratory  centre  from  some  part  of  the 
body  such  as  the  skin;  the  dotted  lines,  a.f.,  indicate  similar  paths  from  the  brain  to 
the  centre.  The  arrows  show  the  direction  in  which  impulses  travel  along  each  nerve 
or  path. 


the  pulmonary  fibres  be  gently  stimulated,  the  respiration 
often  becomes  hurried.  Thus,  we  have  in  the  vagus  a  nerve 
such  that  impulses  passing  up  it  may  quicken  the  respiration 
by  their  action  on  the  respiratory  centre. 


184  ELEMENTARY   PHYSIOLOGY  less 

If  on  the  other  hand  the  branch  of  the  vagus  supplying 
the  larynx,  the  superior  laryngeal  nerve,  be  cut,  and  its 
central  end  be  stimulated,  the  result  is  that  the  respiration 
may  be  sloived,  even  to  a  complete  cessation  of  all  respiratory 
movements. 

In  the  case  of  the  vagus,  impulses  seem  to  be  ordinarily 
always  passing  up  it  from  the  lungs  to  the  respiratory  centre, 
for  if  the  vagus  nerves  be  simply  cut,  the  respiration  be- 
comes at  once  extremely  slow,  and  remains  so. 

These  two  nerves  without  doubt  act  in  life  as  they  act 
upon  artificial  stimulation  and  may  be  taken  as  typical  of 
their  kind,  the  one  quickening,  the  other  slowing  the 
respiration.  But  similar  nerves  run  to  the  respiratory 
centre  from  all  parts  of  the  body,  notably  from  the  skin, 
also  from  the  brain,  and  by  their  varied  action  largely 
determine  the  action  of  the  centre,  and  thus  the  manifold 
changes  which  the  respiratory  movements  from  time  to  time 
undergo. 

11.  Influence  of  Blood-supply  on  the  Respiratory  Cen- 
tre. Dyspnoea  and  Asphyxia. —  The  function  of  respiration 
has  for  its  one  great  object  the  conversion  of  venous  into 
arterial  blood.  Hence  we  might  expect  that  the  mechanism 
which  controls  it  should  be  adjusted  so  as  to  be  extremely 
sensitive  to  the  varying  condition  of  the  blood.  This  expec- 
tation is  justified  by  facts,  for,  although  the  respiratory  centre 
is  keenly  responsive  to  impulses  brought  to  bear  upon  it 
along  various  nerves,  it  is  even  more  so  to  the  influence 
exerted  by  the  varying  quality  of  the  blood  circulating  in  the 
capillaries  of  the  spinal  bulb.  Thus,  when  by  any  means  the 
blood  becomes  less  arterialized  than  it  should  be,  the  res- 
piratory centre  feels  this  change,  and  is  at  once  stimulated 
to  greater  activity  in  the  endeavour,  by  an  increased  force 
and  frequency  of  the  respiratory  movements,  to  restore  the 


v  INFLUENCE  OF   BLOOD-SUPPLY  185 

blood  to  its  proper  condition.  In  other  words,  venous 
blood  makes  the  respiratory  centre  work  faster  and  more 
vigorously. 

The  blood  becomes  more  venous  whenever  the  free  access 
of  air  to  the  lungs  is  interfered  with  ;  as,  for  instance,  when 
a  man  is  strangled,  drowned,  or  choked  by  food  or  other 
obstacle  in  the  trachea.  But  the  blood  may  become  unusu- 
ally venous  by  means  less  violent  than  the  above.  Since 
the  rapidity  of  diffusion  between  two  gaseous  mixtures  de- 
pends on  the  difference  of  the  proportions  in  which  their 
constituents  are  mixed,  it  follows  that  the  more  nearly  the 
composition  of  the  tidal  air  approaches  that  of  the  stationary 
air,  the  slower  will  be  the  diffusion  of  oxygen  inwards,  and 
of  carbonic  acid  outwards,  and  the  more  deficient  in  oxygen 
and  overcharged  with  carbonic  acid  will  the  air  in  the  alveoli 
become.  Thus,  by  breathing  in  a  confined  space,  the  oxy- 
gen in  the  tidal  air  is  gradually  diminished  and  the  carbonic 
acid  gradually  increased  until  at  length  a  point  is  reached 
when  the  change  effected  in  the  stationary  air  is  too  slight 
to  enable  it  to  supply  the  pulmonary  blood  with  oxygen, 
and  to  relieve  it  of  carbonic  acid  to  the  extent  required  for 
its  proper  arterialisation. 

"When  from  any  of  the  above  causes  the  blood  sent  to  the 
respiratory  centre  is  more  venous  than  usual,  the  centre  is 
stimulated  and  the  respiratory  movements  become  quicker 
nnd  more  forcible.  This  condition  is  usually  spoken  of  as 
dyspnoea,  or  laboured  breathing.  It  is  characterised  by  the 
increased  force  and  frequency  with  which  both  the  inspi- 
ratory and  expiratory  muscles  contract.  If  the  offending 
cause  of  dyspnoea  be  not  removed,  the  blood  becomes  more 
and  more  venous.  By  this  means  the  respiratory  centre 
is  spurred  on  to  still  greater  activity.  Not  only  do  the 
ordinary  muscles  of  respiration  contract  more  vigorously, 


1 86  ELEMENTARY   PHYSIOLOGY  less. 

but  the  accessory  muscles  (p.  169)  come  into  more  promi- 
nent play,  and  chiefly  those  which  assist  expiration.  Still 
later,  nearly  all  the  muscles  of  the  body  are  thrown  into  a 
state  of  violent  contracting  activity,  and  with  the  onset  of 
these  convulsions  dyspnoea  passes  over  into  asphyxia.  The 
violence  of  the  convulsive  movements  speedily  leads  to 
exhaustion,  and  the  convulsions  cease.  After  this  stage  is 
reached,  a  long-drawn  inspiration  takes  place  at  intervals ; 
but  the  intervals  become  longer  and  longer  and  the  inspira- 
tory movements  more  and  more  feeble  until  the  last  breath 
is  taken  and  breathing  ends  with  an  expiratory  gasp.1 

Venous  blood  is  distinguished  from  arterial  by  two  fea- 
tures, by  having  less  oxygen  and  more  carbonic  acid.  Hence, 
in  asphyxia,  two  influences  of  a  distinct  nature  are  cooper- 
ating ;  one  is  the  deprivation  of  oxygen,  the  other  is  the 
excessive  accumulation  of  carbonic  acid  in  the  blood.  Oxy- 
gen starvation  and  carbonic  acid  poisoning,  each  of  which  is 
injurious  in  itself,  are  at  work  together;  but  of  these,  the 
lack  of  oxygen  is  the  real  cause  of  asphyxia. 

The  effects  of  oxygen  starvation  may  be  studied  sepa- 
rately, by  placing  a  small  animal  under  the  receiver  of  an 
air-pump  and  exhausting  the  air ;  or  by  replacing  the  air  by 
a  stream  of  hydrogen  or  nitrogen  gas.  In  these  cases  no 
accumulation  of  carbonic  acid  is  permitted,  but,  on  the 
other  hand,  the  supply  of  oxygen  soon  becomes  insufficient, 
and  the  animal  quickly  dies  with  all  the  symptoms  of  as- 
phyxia. And  if  the  experiment  be  made  in  another  way,  by 
placing  a  small  mammal,  or  bird,  in  air  from  which  the  car- 
bonic acid  is  removed  as  soon  as  it  is  formed,  the  animal  will 
nevertheless  die  asphyxiated  as  soon  as  the  amount  of  oxy- 
gen is  reduced  to  10  per  cent,  or  thereabouts. 

1  The  term  asphyxia  is  sometimes  used  to  include  all  the  stages,  from 
the  onset  of  dyspnoea  until  death  ensues. 


v  INFLUENCE   OF   BLOOD-SUPPLY  185 

The  directly  poisonous  effect  of  carbonic  acid,  on  the 
other  hand,  has  been  very  much  exaggerated.  A  very  large 
quantity  of  pure  carbonic  acid  (10  to  15  or  20  per  cent.) 
may  be  contained  in  air,  without  producing  any  very  serious 
immediate  effect,  if  the  quantity  of  oxygen  be  simultaneously 
increased. 

Moreover,  such  symptoms  as  do  occur  when  the  carbonic 
acid  in  the  air  breathed  is  increased  without  any  corre- 
sponding decrease  in  the  oxygen,  are  not  exactly  those  of 
asphyxia  but  are  said  to  resemble  rather  those  of  narcotic 
poisoning.  So  that  the  chief  cause  of  asphyxia  in  strangling, 
drowning,  or  choking,  or  however  produced,  is  the  diminu- 
tion of  the  oxygen  in  the  air  of  the  lungs  and  consequently 
a  diminution  of  the  oxygen  in  the  blood. 

And  that  it  is  the  lack  of  oxygen  which  is  the  important 
thing  is  further  shown  by  the  asphyxiating  effects  of  certain 
poisonous  gases.  Thus  sulphuretted  hydrogen,  so  well  known 
by  its  offensive  smell,  has  long  had  the  repute  of  being  a 
positive  poison.  But  its  evil  effects  appear  to  arise  chiefly, 
if  not  wholly,  from  the  circumstance  that  its  hydrogen  com- 
bines with  the  oxygen  carried  by  the  blood-corpuscles,  and 
thus  gives  rise,  indirectly,  to  a  form  of  oxygen  starvation. 

Carbonic  oxide  gas  (carbon  monoxide,  CO)  has  a  much 
more  serious  effect,  as  it  turns  out  the  oxygen  from  the 
blood-corpuscles,  and  forms  a  very  stable  combination  of 
its  own  with  the  haemoglobin.  The  compound  thus  formed 
is  only  very  gradually  decomposed  by  fresh  oxygen,  so  that, 
if  any  large  proportion  of  the  blood-corpuscles  be  thus  ren- 
dered useless,  the  animal  dies  before  restoration  can  be 
effected.  Badly  made  common  coal  gas  sometimes  con- 
tains 20  to  30  per  cent,  of  carbon  monoxide ;  and,  under 
these  circumstances,  a  leakage  of  the  pipes  in  a  house  may 
be  extremely  perilous  to  life. 


1 88  ELEMENTARY   PHYSIOLOGY  less. 

12.   The  Influence  of  Respiration  on  the  Circulation.  — 

Just  as  there  are  certain  secondary  phenomena  which 
accompany,  and  are  explained  by,  the  action  of  the  heart, 
so  there  are  secondary  phenomena  which  are  similarly 
related  to  the  working  of  the  respiratory  apparatus.  Of 
these  the  chief  is  the  effect  of  the  inspiratory  and  expira- 
tory movements  upon  the  circulation. 

In  consequence  of  the  elasticity  of  the  lungs,  a  certain 
force  must  be  expended  in  distending  them,  and  this  force 
is  found  experimentally  to  become  greater  and  greater  the 
more  the  lung  is  distended ;  just  as,  in  stretching  a  piece 
of  india-rubber,  more  force  is  required  to  stretch  it  a  good 
deal  than  is  needed  to  stretch  it  only  a  little.  Hence,  when 
inspiration  takes  place,  and  the  lungs  are  distended  with 
air,  the  heart  and  the  great  vessels  in  the  chest  are  sub- 
jected to  a  less  pressure  tKan  are  the  blood-vessels  of  the 
rest  of  the  body. 

For  the  pressure  of  the  air  contained  in  the  lungs  is 
exactly  the  same  as  that  exerted  by  the  atmosphere  upon 
the  surface  of  the  body  ;  that  is  to  say,  fifteen  pounds  on 
the  square  inch.  But  a  certain  amount  of  this  pressure 
exerted  by  the  air  in  the  lungs  is  counterbalanced  by  the 
elasticity  of  the  distended  lungs.  Say  that  in  a  given  con- 
dition of  inspiration  a  pound l  pressure  on  the  square  inch 
is  needed  to  overcome  this  elasticity,  then  there  will  be 
only  fourteen  pounds  pressure  on  every  square  inch  of  the 
heart  and  great  vessels.  And  hence  the  pressure  on  the 
blood  in  these  vessels  will  be  one  pound  per  square  inch 
less  than  that  on  the  veins  and  arteries  of  the  rest  of  the 
body,  which  lie  outside  the  thorax.  If  there  were  no 
aortic,  or  pulmonary,  valves,  and  if   the    structure   of  the 

1  A  "pound"  is  stated  here  for  simplicity's  sake.  As  a  matter  of  fact 
the  pressure  required  is  much  less  than  this,  not  more  than  2  or  3  ounces, 


v  THE   INFLUENCE   OF   RESPIRATION  1S9 

vessels,  and  the  pressure  upon  the  blood  in  them,  were 
everywhere  the  same,  the  result  of  this  excess  of  pressure 
on  the  surface  would  be  to  drive  all  the  blood  from  the 
arteries  and  veins  and  the  rest  of  the  body  into  the  heart 
and  great  vessels  contained  in  the  thorax.  And  thus  the 
diminution  of  the  pressure  upon  the  thoracic  blood-cavities 
produced  by  inspiration  would,  practically,  suck  the  blood 
from  all  parts  of  the  body  towards  the  thorax.  But  the 
suction  thus  exerted,  while  it  hastened  the  flow  of  blood 
to  the  heart  in  the  veins,  would  equally  oppose  the  flow 
from  the  heart  to  the  arteries,  and  the  two  effects  might 
balance  one  another. 

As  a  matter  of  fact,  however,  we  know  — 

(1)  That  the  blood  in  the  great  arteries  is  constantly 
under  a  very  considerable  pressure,  exerted  by  their  elastic 
walls  ;  while  that  of  the  veins  is  under  little  pressure. 

(2)  That  the  walls  of  the  arteries  are  strong  and  resist- 
ing, while  those  of  the  veins  are  weak  and  flabby. 

(3)  That  the  veins  have  valves  opening  towards  the 
heart ;  and  that,  during  the  diastole,  there  is  no  resistance 
of  any  moment  to  the  free  passage  of  blood  into  the  heart ; 
while,  on  the  other  hand,  the  cavity  of  the  arteries  is  shut 
off  from  that  of  the  ventricle,  during  the  diastole,  by  the 
closure  of  the  semilunar  valves. 

Hence  it  follows  that  equal  pressures  applied  to  the 
surface  of  the  veins  and  to  that  of  the  arteries  must  pro- 
duce very  different  effects.  In  the  veins  the  pressure  is 
something  which  did  not  exist  before  ;  and  partly  from  the 
presence  of  valves,  partly  from  the  absence  of  resistance  in 
che  heart,  partly  from  the  presence  of  resistance  in  the 
capillaries,  it  all  tends  to  accelerate  the  flow  of  blood 
towards  the  heart.  In  the  arteries,  on  the  other  hand,  the 
pressure  is  only  a  fractional  addition  to  that  which  existed 


190  ELEMENTARY   PHYSIOLOGY  less. 

before  ;  so  that,  during  the  systole,  it  only  makes  a  com- 
paratively small  addition  to  the  resistance  which  has  to 
be  overcome  by  the  ventricle  ;  and  during  the  diastole,  it 
superadds  itself  to  the  elasticity  of  the  arterial  walls  in 
driving  the  blood  onwards  towards  the  capillaries,  inas- 
much as  all  progress  in  the  opposite  direction  is  stopped 
by  the  semilunar  valves. 

It  is,  therefore,  clear,  that  the  inspiratory  movement,  on 
the  whole,  helps  the  heart,  inasmuch  as  its  general  result 
is  to  drive  the  blood  the  way  that  the  heart  propels  it. 

In  expiration,  the  difference  between  the  pressure  of  the 
atmosphere  on  the  surface,  and  that  which  it  exerts  on  the 
contents  of  the  thorax  through  the  lungs,  becomes  less  and 
less  in  proportion  to  the  completeness  of  the  expiration. 
Whenever,  by  the  ascent  of  the  diaphragm  and  the  descent 
of  the  ribs,  the  cavity  of  the  thorax  is  so  far  diminished  that 
pressure  is  exerted  on  the  great  vessels,  the  veins,  owing  to 
the  thinness  of  their  walls,  are  especially  affected,  and  a 
check  is  given  to  the  flow  of  blood  in  them,  which  may 
become  visible  as  a  venous  pulse  in  the  great  vessels  of  the 
neck.  In  its  effect'  on  the  arterial  trunks,  expiration,  like 
inspiration,  is,  on  the  whole,  favourable  to  the  circulation ; 
the  increased  resistance  to  the  opening  of  the  valves  during 
the  ventricular  systole  being  more  than  balanced  by  the 
advantage  gained  in  the  addition  of  the  expiratory  press- 
ure to  the  elastic  reaction  of  the  arterial  walls  during  the 
diastole. 

When  the  skull  of  a  living  animal  is  laid  open  and  the 
brain  exposed,  the  cerebral  substance  is  seen  to  rise  and 
fall  synchronously  with  the  respiratory  movements ;  the  rise 
corresponding  with  expiration,  and  being  caused  by  the 
obstruction  thereby  offered  to  the  flow  of  the  blood  in  the 
veins  of  the  head  and  neck. 


v  VENTILATION  19 1 

The  effects  of  the  respiratory  movements  are  the  same 
[or  the  thoracic  duct.  At  inspiration  the  reduction  of 
pressure  on  the  outside  of  the  duct  draws  lymph  up  into  it 
from  the  abdominal  lymphatic  vessels.  At  expiration,  the 
lymph  cannot  pass  down  again,  owing  to  the  valves  in 
the  duct,  and  is  therefore  sent  on  towards  the  junction  of 
the  latter  with  the  venous  system.  Hence  the  respiratory 
movements  are  a  not  unimportant  aid  to  the  onward  flow 
of  lymph  (see  p.  118). 

13.  Ventilation.  —  In  the  case  of  breathing  the  same  air 
over  and  over  again,  the  deprivation  of  oxygen,  and  the  accu- 
mulation of  carbonic  acid,  cause  injury,  long  before  any 
signs  of  even  dyspnoea  are  observed.  Under  these  circum- 
stances uneasiness  and  headache  arise  when  less  than  1  per 
cent,  of  the  oxygen  of  the  air  is  replaced  by  other  matters ; 
the  symptoms  in  this  case,  however,  are  due  not  so  much  to 
the  diminution  of  oxygen  or  the  increase  of  carbonic  acid, 
as  to  the  poisonous  effects  of  the  various  organic  matters 
present  in  expired  air  which,  though  existing  in  minute  quan- 
tities, have  a  powerfully  deleterious  action.  It  need  hardly 
be  added  that  the  persistent  breathing  of  such  air  tends  to 
lower  all  kinds  of  vital  energy,  and  predisposes  to  disease. 
Hence  the  necessity  of  sufficient  air  and  of  ventilation  for 
every  human  being. 

The  object  of  ventilation  is  to  prevent  the  accumulation 
of  these  organic  impurities  (p.  174)  and  any  deficiency  of 
oxygen,  such  as  may  arise  from  burning  gas  in  a  room  for 
purposes  of  illumination.  Since  the  organic  matter  does 
not  admit  of  direct  estimation,  the  percentage  of  carbonic 
acid  in  the  air  is  usually  taken  as  an  indirect  measure  of  its 
amount,  and  this  is  at  the  same  time  a  measure  of  the  defi- 
ciency of  oxygen.  Air  which  has  been  fouled  by  breathing 
is  injurious  if  it  contains  more  than  .05  per  cent,  of  carbonic 


i92  ELEMENTARY   PHYSIOLOGY  less,  v 

acid.  If  the  percentage  of  carbonic  acid  is  to  be  kept  down 
to  this  limit,  a  man  should  live  in  a  room  whose  capacity  is 
not  less  than  28,000  litres  (1,000  cubic  feet)  and  into  which 
at  least  60,000  litres  (2,000  cubic  feet)  of  fresh  air  are 
admitted  each  hour.1 

]  A  cubical  room  ten  feet  high3  wide,  and  long  contains  one  thousand 
cubic  feet  of  air. 


LESSON  VI 

THE  SOURCES  OF  LOSS  AND  OF  GAIN  TO  THE 
BLOOD 

1.  General  Review  of  the  Gain  and  Loss.  — The  blood 
which  has  been  aerated,  or  arterialised,  by  the  process  de- 
scribed in  the  preceding  Lesson,  is  carried  from  the  lungs 
by  the  pulmonary  veins  to  the  left  auricle,  and  is  then  forced 
by  the  auricle  into  the  ventricle,  and  by  the  ventricle  into 
the  aorta.  As  that  great  vessel  traverses  the  thorax,  it  gives 
off  several  large  arteries,  by  means  of  which  blood  is  distrib- 
uted to  the  head,  the  arms,  and  the  walls  of  the  body. 
Passing  through  the  diaphragm  (Fig.  47,  Ad),  the  aortic 
trunk  enters  the  cavity  of  the  abdomen,  and  becomes  what 
is  called  the  abdominal  aorta,  from  which  vessels  are  given 
off  to  the  viscera  of  the  abdomen.  Finally,  the  main  stream 
of  blood  flows  into  the  iliac  arteries,  whence  the  viscera  of 
the  pelvis  and  the  legs  are  supplied. 

Having  in  the  various  parts  of  the  body  traversed  the 
ultimate  ramifications  of  the  arteries,  the  blood,  as  we  have 
seen,  enters  the  capillaries.  Here  the  products  of  the  waste 
of  the  tissues  constantly  pour  into  it ;  and,  as  the  blood  is 
everywhere  full  of  corpuscles,  which,  like  all  other  living 
things,  decay  and  die,  the  products  of  their  decomposition 
also  tend  to  accumulate  in  it,  but  these  are  insignificant 
compared  to  those  coming  from  the  great  mass  of  the 
o  193 


i94  ELEMENTARY   PHYSIOLOGY  less. 

tissues.  It  follows  that,  if  the  blood  is  to  be  kept  pure,  the 
waste  matters  thus  incessantly  poured  into  or  generated  in 
it  must  be  as  constantly  got  rid  of,  or  excreted. 

Three  distinct  sets  of  organs  are  especially  charged  with 
this  office  of  continually  removing  or  "  excreting  "  waste 
matters  from  the  blood.  They  are  the  lungs,  the  kidneys, 
and  the  skin.  These  three  great  organs  may  therefore  be 
regarded  as  so  many  drains  from  the  blood  —  as  so  many 
channels  by  which  it  is  constantly  losing  substance. 

On  the  other  hand,  the  blood,  as  it  passes  through  the 
capillaries,  is  constantly  giving  up  material  by  exudation 
through  the  capillary  walls  into  the  surrounding  tissues,  in 
order  to  supply  them  with  nourishment,  and  thus  in  this 
way  also  is  constantly  losing  matter. 

The  material  which  the  blood  loses  by  giving  it  up  to  the 
tissues  consists  of  complex  organic  bodies,  such  as  proteids, 
fats,  carbohydrates,  and  various  substances  manufactured  out 
of  these,  of  certain  salts,  of  a  large  quantity  of  water,  and 
lastly  of  oxygen. 

The  material  which  the  blood  loses  by  giving  it  up  to  the 
skin,  lungs,  and  kidneys,  passes  away  from  these  organs  as 
water,  as  carbonic  acid,  as  peculiar  organic  substances,  of 
which  one,  called  urea,  is  much  more  abundant  than  the 
others,  and  as  certain  inorganic  salts.  Speaking  generally, 
we  may  say  that  these  organs  together  excrete  from  the 
blood,  water,  carbonic  acid,  urea,  and  salts. 

Another  kind  of  loss  takes  place  from  the  surface  of  the 
body  generally,  and  from  the  interior  of  the  air-passages. 
Heat  is  constantly  being  given  off  from  the  former  by  radia- 
tion, evaporation,  and  conduction  :  from  the  latter,  chiefly 
by  evaporation  ;  and  the  loss  of  heat  in  each  case  is  borne 
by  the  blood  passing  through  the  skin  and  air-passages  re- 
spectively.    Besides  this  a  certain  quantity  of  heat  is  lost  by 


vi  REVIEW   OF  THE   GAIN    AND   LOSS  195 

the  urine  and  faeces,  which  are  always  warm  when  they  leave 
the  body. 

On  the  side  of  gain  we  have,  in  the  first  place,  the  various 
substances  which  are  the  products  of  the  activity  of  the  sev- 
eral tissues,  muscles,  brain,  glands,  etc.,  and  which  pass  from 
the  tissues  into  the  blood.  We  may  speak  of  these  as  waste 
products,  and  one  of  them;  which  is  produced  by  all  the  tis- 
sues, namely,  carbonic  acid,  is  emphatically  a  waste  product 
and  is  got  rid  of  as  soon  as  possible.  But  some  of  the  sub- 
stances which  are  returned  to  the  blood  from  the  tissues  are 
not  wholly  useless  matters  to  be  thrown  off  as  rapidly  as 
possible  ;  they  are  capable  of  being  used  up  again  by  some 
tissue  or  other.  Thus,  as  we  shall  see,  the  liver,  at  certain 
times  at  all  events,  returns  to  the  blood  a  certain  quantity 
of  sugar,  which  is  made  use  of  in  other  parts  of  the  body, 
and  similarly  the  spleen,  while  it  takes  up  certain  substances 
from  the  blood,  gives  back  to  the  blood  certain  other  sub- 
stances, which  we  can  hardly  speak  of  as  waste  matters  in 
the  sense  of  being  useless  material  fit  only  to  be  at  once 
thrown  away. 

In  the  second  place,  the  blood  is  continually  receiving 
from  the  alimentary  canal  the  materials  arising  from  the 
food  which  has  been  digested  there.  As  we  shall  see, 
some  of  this  material  passes  directly  from  the  cavity  of 
the  alimentary  canal  into  the  blood,  but  some  of  it  goes 
in  a  more  roundabout  way  through  the  lacteals  or  lym- 
phatics. On  its  way  to  the  blood,  this  latter  is  joined  by 
material  which,  escaping  from  the  blood  and  not  used  by 
the  tissues,  or  passing  from  the  tissues  directly  into  the 
lymphatics,  is  carried  back  to  the  blood  by  the  thoracic 
duct  (see  p.   in). 

In  the  third  place,  the  blood  is  continually  gaining  oxy- 
gen from  the  air,  through  the  lungs. 


i96  ELEMENTARY   PHYSIOLOGY  less. 

Then  again  the  blood,  while  it  loses  heat  by  the  skin  and 
lungs,  gains  heat  from  the  tissues.  As  we  have  already  seen 
(p.  24),  oxidation  is  continually  going  on  in  various  parts 
of  the  body,  and  by  this  oxidation  heat  is  continually  being 
set  free.  Some  of  this  oxidation  may  take  place  in  the 
blood  itself ;  we  do  not  know  exactly  how  much,  but  prob- 
ably very  little.  The  greater  part  of  the  heat  is  generated 
in  the  tissues,  in  the  muscles,  and  elsewhere,  and  is  given 
up  by  the  tissues  to  the  blood.  So  that  we  may  say  that 
the  blood  gains  heat  from  the  tissues. 

These  several  gains  and  losses  are  for  the  most  part 
going  on  constantly,  but  are  greater  at  one  time  than  at 
another.  Thus  the  gain  to  the  blood  from  the  alimentary 
canal  is  much  greater  some  time  after  a  meal  than  just 
before  the  next  meal,  though,  unless  the  meals  be  very  far 
apart  indeed,  the  whole  of  the  material  of  one  meal  has 
not  passed  into  the  blood  before  the  next  meal  is  begun. 
Again,  though  the  muscles,  even  when  completely  at  rest, 
are  taking  up  oxygen  and  nutritive  material,  and  giving  out 
carbonic  acid  and  other  waste  products,  they  give  out  and 
take  in  much  more  when  they  are  at  work.  So  also  cer- 
tain "  secreting  glands,"  as  they  are  called,  which  we  shall 
study  presently,  such  as  the  salivary  glands,  have  periods 
of  repose  ;  it  is  at  certain  times  only,  as  when  food  has 
been  taken,  that  they  pour  out  any  appreciable  quantity  of 
fluid.  Hence,  though  they  are  probably  taking  up  material 
from  the  blood  and  storing  it  up  in  their  substance  even 
when  they  appear  at  rest,  they  take  up  much  more  and 
so  become  much  more  distinctly  means  of  loss  to  the 
blood  when  they  are  actively  pouring  out  their  secretions. 
In  the  case  of  the  liver,  the  loss  to  the  blood  is  more 
constant,  since  the  secretion  of  bile,  as  we  shall  see,  is  con- 
tinually going  on,  though  greater  at  certain  times  than  at 


vi  SOURCES   OF   LOSS   AND   GAIN  197 

others ;  and  the  materials  for  the  bile  have  to  be  pro- 
vided by  the  blood.  Some  of  the  constituents  of  the  bile, 
however,  pass  back  from  the  intestines  into  the  blood ;  and 
so  far  the  loss  to  the  blood  by  the  liver  is  temporary  only. 

Of  all  the  gains  to  the  blood,  perhaps  the  most  constant 
is  that  of  oxygen,  and  of  all  the  losses,  perhaps  the  most 
constant  is  that  of  carbonic  acid ;  but  even  these  vary  a 
good  deal  at  different  times  or  under  different  circum- 
stances. 

Broadly  speaking,  then,  the  blood  gains  oxygen  from  the 
lungs,  complex  organic  food  materials  from  the  alimentary 
canal,  and  various  substances,  which  we  may  speak  of  as 
waste  substances,  from  the  several  tissues ;  and  it  loses, 
on  the  one  hand,  material,  which  we  may  speak  of  as  con- 
structive material,  to  the  several  tissues ;  and,  on  the  other 
hand,  material  which  passes  away  by  the  skin,  lungs,  and 
kidneys,  as  water,  carbonic  acid,  urea,  and  saline  bodies. 

And  while  it  is  continually  receiving  heat  from  the  sev- 
eral tissues,  it  is  also  continually  losing  heat  by  the  skin, 
lungs,  and  other  free  surfaces  of  the  body. 

The  sources  of  loss  and  gain  to  the  blood  may  be 
conveniently  arranged  in  the  following  tabular  form  :  — 

Sources  of  Loss  and  Gain  to  the  Blood1 

A.   Sources  of  Loss  :  — 
I.    Loss  of  Matter. 

1.  The    lungs:    carbonic   acid    and   water    (fairly 
constant). 

1  The  learner  must  be  careful  not  to  confound  the  losses  and  gains 
of  the  blood  with  the  losses  and  gains  of  the  body  as  a  whole.  The  two 
differ  in  much  the  same  way  as  the  internal  commerce  of  a  country  differs 
from  its  export  and  import  trade. 


198  ELEMENTARY    PHYSIOLOGY  less, 

2.  The  kidneys  :    urea,  water,  salines   (fairly  con- 

stant) . 

3.  The  skin:  water,  salines  (fairly  constant). 

4.  The  tissues  :  constructive  material  (variable,  es- 

pecially in  the  case  of  those  tissues  whose 
activity  is  intermittent,  such  as  the  muscles, 
many  secreting  glands,  etc.). 

II.    Loss  of  Heat. 

1.  The  skin. 

2.  The  lungs. 

3.  The  excretions  by  the  kidney  and  the  alimen- 

tary canal. 

B.   Sources  of  Gain  :  — 
I.    Gain  of  Matter. 

1.  The  lungs:  oxygen  (fairly  constant). 

2.  The  alimentary  canal :  food  (variable). 

3.  The   tissues  :    products  of  their  activity,  waste 

matters  (always  going  on  but  varying 
according  to  the  activity  of  the  several 
tissues). 

4.  The   lymphatics  :    lymph   (always  going  on  but 

varying  according  to  the  activity  of  the 
several  tissues).1 

II.    Gain  of  Heat. 

1.  The    tissues  generally,  especially  the  more  ac- 

tive ones,  such  as  the  muscles. 

2.  The    blood    itself,    probably   to   a   very   small 

extent. 


1  The  gain   from   (hose   lymphatics  which   are   called   lacteals,  since  it 
:omes  from  the  lalimeniary  canal,  varies  much  more. 


in  SECRETION   IN   GENERAL  195 

2.  Secretion  in  General.  —  Secreting  glands  have  been 
spoken  of  as  sources  of  loss  and  gain  to  the  blood.  A  brief 
and  general  survey  of  their  structure  and  mode  of  action 
may  profitably  be  made  here.  In  principle,  they  are  nar- 
row pouches  of  mucous  membrane,  or  of  the  integument  of 
the  body,  lined  by  a  continuation  of  the  epithelium,  or 
the  epidermis  (Fig.  57).  According  as  the  pouch  has  the 
form  of  a  tube  or  is  dilated,  the  gland  is  said  to  be  tubular  ( 1 ) 
or  saccular  (3).  Forms  intermediate  between  these  two 
are  not  uncommon.  When  a  single  pouch  exists,  the  gland 
is  called  simple  ;  when  divided  into  two  or  more  pouches, 
it  is  compound.  Compound  saccular  glands  are  usually 
termed  racemose  (6),  from  their  fancied  resemblance  to  a 
bunch  of  grapes.  The  neck  by  which  the  gland  communi- 
cates with  the  free  surface  of  the  mucous  membrane  or  skin 
is  called  its  duct  (//).  The  epithelium  lining  the  gland  con- 
stitutes the  secreting  portion.  It  is  composed  of  conspicu- 
ous, characteristic  cells,  bathed  over  their  attached  surfaces 
by  lymph  and  surrounded  closely  by  a  rich  network  of  capil- 
laries, Frequently  a  thin,  inconspicuous  membrane  of  fiat 
cells,  the  basement  membrane  (fi),  lies  immediately  outside 
the  secreting  cells.  The  manifest  function  of  the  secreting 
cells  is  to  receive  from  the  blood,  through  the  lymph,  water, 
salts,  and  other  substances,  to  manufacture  from  these  raw- 
materials  certain  specific  chemical  substances,  and  finally 
to  pass  out  through  the  duct  to  the  free  surface  the  result- 
ing mixture  of  water,  salts,  and  specific  substances,  as  the 
secretion.1 

1  The  word  "  secretion  "  is  used  by  physiologists  in  three  senses.  Pri- 
marily it  is  used  to  denote  the  sum  total  of  the  processes  by  which  a  gland 
or  organ  forms  the  fluid  which  it  gives  out ;  thus  we  say  that  the  salivary 
glands  "  secrete  "  saliva.  Further,  it  often  signifies  merely  the  process  of 
extrusion  of  the  fluid  from  the  gland  in  which  it  is  formed.  Lastly,  the 
fluid  is  itself  spoken  of  as  "a  secretion."     The  word  "excretion"  is  usu 


ELEMENTARY   PHYSIOLOGY 
A 


Fig.  57.  —  A  Diagram  to  illustrate  the  Structure  of  Glands. 
A.  typical  structure  of  a  mucous  membrane;  a,  the  layer  of  epithelium  cells; 
6,  the  basement  membrane;  c,  the  dermis,  with  e,  a  blood-vessel,  andy,  connective 
tissue  corpuscles. 

1.  A  simple  tubular  gland;  letters  the  same  as  in  A. 

2.  A  tubular  gland  divided  at  its  base.     In  this  and  succeeding  figures  the  blood 
vessels  are  omitted. 

3.  A  simple  saccular  gland. 

4.  A  divided  saccular  gland,  with  a  duct,  d. 

5.  A  similar  gland  still  more  divided. 

6.  A  racemose  gland,  part  only  being  drawn. 


vi  THE   URINARY   ORGANS  aoi 

A  less  obvious  but  not  less  important  function  of  manv 
glands  is  that  of  giving  to  the  blood  material  which  is  thus 
passed  on  to  other  glands  for  excretion  or  can  be  made  use 
of  by  other  parts  of  the  body.  This  property  of  internal 
secretion,  which  has  become  well  recognised  only  and  is  not 
yet  fully  elucidated,  belongs  prominently  to  the  liver  and 
certain  so-called  "  ductless  glands,"  such  as  the  thyroid  body 
and  the  suprarenal  bodies. 

In  the  preceding  Lesson  we  have  described  the  operation 
by  which  the  lungs  withdraw  from  the  blood  much  carbonic 
acid  and  water,  and  supply  oxygen  to  the  blood.  In  this 
and  the  succeeding  Lesson  some  other  of  the  chief  sources 
of  loss  and  of  gain  to  the  blood  will  be  discussed  in  detail. 

3.  The  Urinary  Organs. — We  now  proceed  to  the  sec- 
ond source  of  continual  loss,  the  Kidneys. 

Of  these  organs  there  are  two,  placed  at  the  back  of  the 
abdominal  cavity,  one  on  each  side  of  the  lumbar  region  of 
the  spine.  Each,  though  somewhat  larger  than  the  kidney 
of  a  sheep,  has  a  similar  shape.  The  depressed,  or  concave, 
side  of  the  kidney  is  turned  inwards,  or  towards  the  spine ; 
and  its  convex  side  is  directed  outwards  (Fig.  58).  From 
the  middle  of  the  concave  side  (called  the  hilus)  of  each 
kidney,  a  long  tube  with  a  small  bore,  the  ureter  ( U) ,  pro- 
ceeds to  the  bladder  (  Vu). 

The  latter,  situated  in  the  pelvis,  is  an  oval  bag,  the  walls 
of  which  contain  abundant  unstriped  muscular  fibre,  while 
it  is  lined,  internally,  by  mucous  membrane,  and  coated 
externally  by  a  layer  of  the  peritoneum,  or  double  bag  of 
serous  membrane,  which  has  exactly  the  same  relations  to 

ally  applied  to  any  fluid  which  after  its  formation  is  useless  and  requires 
to  be  at  once  got  rid  of.  Tims,  we  say  that  urine  is  an  excretion  which 
is  secreted  {i.e.  formed)  by  the  kidneys;  ami  we  speak  of  those  secretory 
structures  which  get  rid  of  waste  as  excretory  organs. 


ELEMENTARY   PHYSIOLOGY 


the  cavity  of  the  abdomen  and  the  viscera  contained  in 
them  as  the  pleurae  have  to  the  thoracic  cavity  and  the 
lungs.  The  ureters  open  side  by  side,  but  at  some  little 
distance  from  one  another,  on  the  posterior  and  inferior 
wall  of  the  bladder.  Each  ureter  is  lined  by  an  epithelium 
consisting  of  several  layers  of  cells.  Outside  of  these  is  a 
muscular  coat  made  up  of  unstriated  muscle-fibres,  arranged 


Fig.   58. —  The  Urinary  Organs  seen  from  behind.      (From  Moore's  Elemen- 
tary Physiology.) 
R,   right   kidney;    U,    ureter;    Vu,    bladder;    Ua,  commencement  of  urethra;    A, 
aorta;   Ar,  right  renal  artery;    Ve,  inferior  vena  cava;    Vr,  right  renal  vein. 

in  three  layers  and  surrounded  externally  by  some  fibrous 
connective  tissue.  In  front  of  the  ureters  is  a  single  aper- 
ture which  leads  into  the  canal  called  the  urethra  (Fig.  58, 
U<i),  by  which  the  cavity  of  the  bladder  is  placed  in  com- 
munication with  the  exterior  of  the  body.  The  openings  of 
the  ureters  enter  the  walls  of  the  bladder  obliquely,  so  that 


VI  THE  STRUCTURE   OF  A  KIDNEY  203 

it  is  much  more  easy  for  the  fluid  to  pass  from  the  ureters 
into  the  bladder  than  for  it  to  get  the  other  way,  from  the 
bladder  into  the  ureters. 

Mechanically  speaking,  there  is  little  obstacle  to  the  free 
flow  of  fluid  from  the  ureters  into  the  bladder,  and  from 
the  bladder  into  the  urethra,  and  so  outwards ;  but  certain 
muscular  fibres  arranged  circularly  around  the  part  called 
the  "  neck  "  of  the  bladder,  where  it  joins  the  urethra,  con- 
stitute what  is  termed  a  sphincter,  and  are  usually,  during  life, 
in  a  state  of  contraction,  so  as  to  close  the  exit  of  the  bladder, 
while  the  other  muscular  fibres  of  the  organ  are  relaxed. 

It  is  only  at  intervals  that  this  state  of  matters  is  reversed  ; 
and  the  walls  of  the  bladder  contracting,  while  its  sphincter 
relaxes,  its  contents,  the  urine,  are  discharged.  But,  though 
the  expulsion  of  the  secretion  of  the  kidneys  from  the  body 
is  thus  intermittent,  the  excretion  itself  is  constant,  and  the 
urinary  fluid  flows,  drop  by  drop,  from  the  opening  of  the 
ureters  into  the  bladder.  Here  it  accumulates,  until  its 
quantity  is  sufficient  to  give  rise  to  the  uneasy  sensations 
which  compel  its  expulsion. 

4.  The  Structure  of  a  Kidney.  —  When  a  longitudinal 
section  of  a  kidney  is  made  (Fig.  59),  the  upper  end  of  the 
ureter  (£/)  seems  to  widen  out  into  a  basin-like  cavity  (P), 
which  is  called  the  pelvis  of  the  kidney.  Into  this  sundry 
conical  elevations,  called  the  pyramids  (Py),  project;  and 
their  summits  present  multitudes  of  minute  openings  —  the 
final  terminations  of  the  uriniferous  tubules,  of  which  the 
mass  of  the  kidney  is  chiefly  made  up.  If  the  tubules 
be  traced  from  their  openings  towards  the  outer  surface, 
they  are  found,  at  first,  to  lie  parallel  with  one  another  in 
bundles,  which  radiate  towards  the  surface,  and  subdivide 
as  they  go ;  but  at  length  they  spread  about  irregularly, 
and  become  coiled  and  interlaced.    From  this  circumstance, 


204 


ELEMENTARY   PHYSIOLOGY 


the  middle  part  or  medulla  {medulla,  marrow)  of  the  kidney 
looks  different  from  the  superficial  part  or  cortex  (cortex., 
bark)  ;  but,  in  addition,  the  cortical  part  is  more  abundantly 
supplied  with  vessels  than  the  medullary,  and  hence  has  a 
darker  aspect.  Each  tubule  after  a  very  devious  course 
ultimately  terminates  in  a  dilatation  (Fig.  60)  called  a 
Malpighian  capsule.  Into  the  summit  of  each  capsule,  a 
small  vessel  (Fig.  60  v. a),  one  of  the  ultimate  branches  of 


Fig.  59.  —  Longitudinal  Section  of  the  Human  Kidney. 

Ct,   the   cortical  substance;    M,  the   medullary   substance;    Py,   the  pyramids;  P, 
the  pelvis  of  the  kidney;    U,  the  ureter;  R,A.  the  renal  artery. 


the  renal  artery,  which  reaches  the  kidney  at  the  concave 
side  with  the  ureters  and  divides  into  branches  which  pass 
in  between  the  pyramids  (Fig.  59,  RA),  enters  (driving  the 
thin  wall  of  the  capsule  before  it),  and  immediately  breaks  up 
into  a  bunch  of  looped  capillaries  called  a  glomerulus  (Fig. 
6o,  gl),  which  nearly  fills  the  cavity  of  the  capsule.     The 


vi  THE   STRUCTURE  OF  A   KIDNEY  205 

blood  is  carried  away  from  this  glomerulus  by  a  small  vein 
or  vessel  (ve),  which  does  not,  at  once,  join  with  other  veins 
into  a  larger  venous  trunk,  but  opens  into  the  network  of 
capillaries  (Fig.  61)  which  surrounds  the  tubule,  thus 
repeating  the  portal  circulation  on  a  small  scale. 


O  ~  " 


Fig.  60.  —  A  Malpighian  Capsule  (highly  magnified). 

v. a,  small  branch  of  renal  artery  entering  the  capsule,  breaking  up  into  the 
glomerulus,^  and  finally  joining  again  to  form  the  vein,  v.e. 

c,  the  uriniferous  tubule;  a,  the  epithelium  over  the  glomerulus;  b,  the  epithelium 
lining  the  capsule. 

The  course  of  the  tubules  is  devious  and  peculiar.  After 
leaving  the  capsule  each  tubule  becomes  twisted  and  is 
spoken  of  as  convoluted  (Fig.  62,  II).  Passing  towards 
the  medulla,  at  first  in  a  slightly  spiral  course,  it  proceeds 
straight  down  into  the  pyramid,  where  it  bends  back  upon 
itself  and  runs  up  again  into  the  cortex.  The  loop  thus 
formed  is  known  as  the  loop  of  Henle,1  and  the  two  parts 
of  which  it  is  formed  are  called  the  descending  limb  and 
the  ascending  limb  of  the  loop    {III,  IV). 

Reaching  the  cortex  once  more  the  tubule  becomes 
irregular  and  then  again  convoluted  {V),  after  which  it 
passes    into    a    straight    part    or    collecting  tubule    ( VI), 

1  Who  first  described  it. 


206 


ELEMENTARY   PHYSIOLOGY 


which,  leaving  the  cortex  finally  for  the  medulla  and  unit- 
ing with  other  similar  collecting  tubules,  forms  the  dis- 
charging tubule  {IX),  which  opens  near  the  summit  of 
a  pyramid.  The  kidney  is  thus  seen  to  be  a  compound 
tubular  gland,  but  a  very  complicated  one. 

Each  tubule  is  lined  throughout  by  epithelial  cells,  and 
these  differ  in  their  characters  in  the  several  parts  of  its 
course.  The  details  of  these  differences  are  numerous 
and  complicated,  but  the  following  statement   includes  all 


Fig.  6i.  —  Circulation  in  the  Kidney. 

ai,  small  branch  of  renal  artery  giving  off  the  branch  va,  which  enters  the 
glomerulus,  gl,  issues  as  ve,  and  then  breaks  up  into  capillaries,  which  after  sur- 
rounding the  tubule  find  their  way  by  v  into  vi,  a  branch  of  the  renal  vein;  6,  parts 
of  the  cortex  where  there  are  glomeruli;  m,  capillaries  around  tubules  in  parts  of  the 
cortical  substance  where  there  are  no  glomeruli.  • 


that  is  most  essential.  The  cells  lining  the  Malpighian 
capsule  and  covering  the  capillaries  of  its  contained  glo- 
merulus are  much  flattened,  and  constitute  an  excessively 
thin  membrane  (Fig.  60,  b,  a),  an  arrangement  which 
appears  to  be  favourable  to  the  ready  passage  of  certain 
constituents  of  the   blood   into  the  cavity  of  the   capsule. 


THE   STRUCTURE   OE   A    KIDNEY 


207 


The  cells  in  the  convoluted,  spiral,  and  irregular  tubules, 
and  some  portions  of  the  loop  of  Henle,  are,  on  the  whole, 
large,  very  granular,  and  striated,  and  both  the  cells  and  their 
nuclei  stain  readily  and  deeply  (Fig.  63,  a).  These  cells 
have,  in  fact,  the  appearance  of  true  secreting  cells,  and 


Fig.    62. 


•  Diagrammatic    View    of   the    Course    of    the    Tubules    in   the 
Kidney. 


r,  cortical  portion  answering  to  Ct  in  Fig.  59,  k  being  close  to  the  surface  of  the 
kidney;  g,  />,  medullary  portion,/  reaching  to  the  summit  of  a  pyramid. 

/,  Malpighian  capsule;  //,  /',  convoluted  tubules;  !I[,  descending  limb,  and 
IV,  ascending  limb,  of  the  loop  of  Henle;  VI,  VII,  VIII,  collecting  tubules; 
IX,  discharging  tubule. 


experiments  show  that  they  are  such.  They  are  surrounded 
by  a  rich  capillary  network  (Fig.  61).  In  the  collecting 
and  discharging  tubules  the  cells  are  cubical  or  columnar 
(Fig.  63,  b),  quite  free  from  granules,  do  not  stain  readily, 
and  apparently  are  not  secretory.     These  portions  of  the 


208 


ELEMENTARY   PHYSIOLOGY 


tubules  are  probably  purely  conducting  in  function.  So 
far  as  the  formation  of  the  urine  is  concerned,  the  impor- 
tant cells  seem  to  be  the  capsular  and  the  secreting  cells. 

The  artery  which  supplies  the  kidney  enters  at  the  hilus 
and  divides  into  branches  which  "pass  around  the  pelvis  and 
proceed  outwards  between  the  pyramids.  At  the  junction 
of  the  medulla  and  cortex  these  branches  spread  out 
sideways  and  form  arches  (Fig.  64).  From  these  arches 
branches  run  (i)  straight  out  to  the  surface  of  the  kidney, 

B 
/•: 


Fig.  63.  —  Types  of  Cells  in  the  Tubules  of  the  Kidney. 

A,  lubules  cut  lengthwise;   B,  tubules  cut  across. 

a,  type  of  (secreting)  cell  lining  the  convoluted,  spiral,  and  irregu.ar  tubules 
b,  type  of  cells  lining  the  collecting  and  discharging  tubules;  n,  nuclei;  c,  in  B, 
capillaries  seen  in  section. 


giving  off  smaller  lateral  branches,  of  which  some  pass  to 
the  capsules  while  others  supply  the  capillary  network  round 
the  tubules  :  (ii)  down  towards  the  pyramids,  in  whose  sub- 
stance they  break  up  into  capillaries.  The  veins  also  form 
arches  at  the  junction  of  the  cortex  and  medulla,  into  which 
the  blood  flows  from  the  capillaries,  and  leave  the  kidney 
by  a  course  parallel  to  that  of  the  entering  arteries. 

5.    The  Urine.  —  The  renal  secretion  is  a  clear  yellowish 
fluid,  whose  specific  gravity  is  not  very  different  from  that 


vi  THE    URINE  209 

of  blood-serum,  being  1.020.  In  health  it  has  a  slightly 
acid  reaction,  due  to  the  presence  of  acid  sodium  phos- 
phate. It  is  composed  chiefly  of  water,  holding  in  solution  : 
(i)  Organic  substances,  of  which  the  chief  is  urea,  with  a 
very  much    smaller  amount    of   uric  acid,      (ii)   Inorganic 


'I 

Fig.  64.  —  Blood-vessels  of  Kidnev.     (Cadiat.) 

a,  part  of  arterial  arch;  b,  interlobular  artery;  c,  glomerulus;  d,  efferent  vessel; 
e,  capillaries  of  cortex;  f,  straight  arteries  of  medulla;  g,  venous  arch;  h,  straight 
veins  of  medulla ;   i,  interlobular  vein. 

salts,  chiefly  sodium  chloride  and  sulphates  and  phosphates 
of  sodium,  potassium,  calcium,  and  magnesium,  (iii)  Col- 
ouring matters,  of  which  but  little  is  known,  (iv)  Gases, 
chiefly  carbonic  acid,  with  a  very  small  amount  of  nitro- 
gen and  still  less  oxygen. 


210  ELEMENTARY   PHYSIOLOGY  less. 

An  average  healthy  man  excretes  about  1,500  c.c.  (3 
pints)  of  urine  each  day.  In  this  are  dissolved  33  grammes 
(ij  oz.  or  about  2  per  cent.)  of  urea  and  not  more  than 
.5  gramme  (8  grains)  of  uric  acid.  The  amount  of  salts 
is  nearly  equal  to  that  of  the  urea,  and  the  larger  part 
consists  of  sodium  chloride. 

The  quantity  and  composition  of  the  urine  vary  greatly 
according  to  the  time  of  day,  the  temperature  and  mois- 
ture of  the  air,  the  fasting  or  replete  condition  of  the  ali- 
mentary canal,  the  nature  of  the  food,  and  the  amount  of 
fluid  consumed. 

The  quantity  depends  on  the  temperature  and  the  mois- 
ture of  the  air  because,  as  we  shall  see  (p.  230),  these 
determine  the  greater  or  less  loss  of  water  by  the  skin, 
and  thus  leave  less  or  more  to  be  excreted  by  the  kidneys. 
The  relationship  of  fluid  consumed  to  the  amount  of  urine 
excreted  is  obvious.  The  composition  varies  with  the  kind 
and  amount  of  food,  chiefly  in  respect  of  the  amount  of 
urea  excreted,  for  the  nitrogen  in  urea  represents  nearly 
all  the  nitrogen  introduced  into  the  body  in  the  proteids. 

This  relationship  of  the  nitrogen  in  food  to  the  nitrogen 
of  urea  confers  upon  urea  its  supreme  importance  as  a 
constituent  of  urine ;  for  the  body  cannot  make  good  its 
nitrogenous  waste  from  any  source  other  than  the  nitrogen 
introduced  into  it  in  the  form  of  proteids.  Hence  varia- 
tions in  the  quantity  of  urea  excreted  thus  become  the 
measure  of  the  amount  of  nitrogen  turned  over  or  "  metabo- 
lised "  in  the  body  from  time  to  time. 

Urea  is  a  white  crystalline  solid,  very  soluble  in  water,  and 
composed  of  carbon,  oxygen,  hydrogen,  and  nitrogen.  Its 
chemical  formula  is  (NH.,)2CO,  from  which  it  is  seen  to 
contain  rather  more  than  46  per  cent,  of  nitrogen. 

Historically,  urea  is  interesting  as  being  the  first  organic 


vi  THE  SECRETION  OF  URINE  211 

animal  product  prepared  (synthetically)  from  inorganic 
sources  (by  Wohler  in   1S28). 

6.  The  Secretion  of  Urine.  —  Many  of  the  constituents 
of  urine  are  present  in  blood.  These  appear  in  the  urine 
dissolved  in  a  large  quantity  of  water,  whereas  many  other 
substances  also  present  in  the  blood  do  not,  in  a  state  of 
health,  make  their  way  into  the  urine.  This  suggests  the 
idea  that  the  kidney  is  a  peculiar  and  delicate  kind  of  filter, 
which  allows  certain  substances  together  with  a  large  quan- 
tity of  water  to  pass  through  it,  but  refuses  to  allow  other 
substances  to  pass  through.  And  when  we  come  to  studv 
the  minute  structure  of  the  kidney,  we  find  much  to  support 
this  idea.  Thus,  we  saw  that  the  surface  of  the  glomerulus 
is,  practically,  in  direct  communication  with  the  exterior  by 
means  of  the  cavity  of  the  tubule  ;  and,  further,  that  in  each 
vessel  of  the  glomerulus  a  thin  stream  of  blood  constantly 
flows,  separated  from  the  cavity  of  the  tubule  only  by  the 
capillary  wall  and  the  very  delicate  epithelial  membrane 
covering  the  glomerulus.  The  Malpighian  capsule  may,  in 
fact,  be  regarded  as  a  funnel,  and  the  membranous  walls 
of  the  glomerulus  as  a  piece  of  very  delicate  but  peculiar 
filtering-paper,  into  which  the  blood  is  poured. 

And  indeed,  though  there  are  some  objections  to  this 
view,  we  have  reason  to  think  that  a  great  deal  of  the  water 
of  urine,  together  with  certain  of  the  constituents  (the  inor- 
ganic salts),  is  thus,  as  it  were,  filtered  off  by  the  Malpighian 
capsules.  But  it  must  be  remembered  that  the  process  is 
after  all  very  different  from  actual  filtering  through  paper ; 
for  filter-paper  will  let  everything  pass  through  that  is  really 
dissolved,  whereas  the  glomerulus,  while  letting  some  things 
through,  refuses  to  admit  others,  even  though  completely 
dissolved.  Filtration  in  the  kidney  acquires  its  peculiarities 
from  the  fact  that,  as  in  the  case  of  lymph-formation  (p.  146), 


2t2  ELEMENTARY    PHYSIOLOGY  less. 

the  filtration  takes  place  through  the  substance  of  living 
cells. 

Speaking  of  the  process,  with  this  caution,  as  one  of  filtra- 
tion, it  is  obvious  that  the  more  full  the  glomerulus  is  of 
blood  the  more  rapid  will  be  the  escape  of  urine.  Hence 
we  find  that  when  blood  flows  freely  to  the  kidney  the  urine 
is  secreted  freely,  but  that  when  the  blood-supply  to  the 
kidney  is  scanty  the  urine  also  is  scanty.  When  the  renal 
nerves  going  to  the  kidney  are  cut,  the  branches  of  the 
renal  artery  dilate,  much  blood  goes  into  the  kidney,  the 
blood-pressure  is  raised  in  the  glomeruli,  and  the  flow  of 
urine  is  copious.  If  the  same  nerves  be  stimulated,  the 
arterial  tubes  are  narrowed  or  constricted,  less  blood  goes 
to  the  kidney,  blood-pressure  is  reduced,  and  the  flow  of 
urine  is  scanty  or  may  be  stopped  altogether. 

We  can  now  explain,  in  part  at  all  events,  how  it  is  that 
the  activity  of  the  kidney  is  influenced  by  the  state  of  the 
skin.  The  quantity  of  blood  in  the  body  being  about  the 
same  at  all  times,  if  a  large  quantity  goes  to  the  skin,  as  in 
warm  weather  and  especially  when  the  skin  is  active  and 
perspiring,  less  will  go  to  the  kidney  and  the  secretion  of 
urine  will  be  small.  On  the  other  hand,  if  the  blood  be 
largely  cut  off  from  the  skin,  as  in  cold  weather,  more  blood 
will  be  thrown  upon  the  kidney  and  more  urine  will  be 
secreted.  Thus  the  skin  and  the  kidneys  play  into  each 
other's  hands  in  their  efforts  to  get  rid  of  the  superfluous 
water  of  the  body. 

But  the  whole  of  the  urine  is  not  thus  excreted,  through  a 
sort  of  filtering  process,  by  the  Malpighian  capsules.  The 
circulation  in  the  kidney  is  peculiar,  inasmuch  as  the  blood 
coming  from  the  glomeruli  is  not  sent  at  once  into  a  vein, 
but  is  carried  into  a  second  capillary  network,  wrapped 
round    the   tubules.     The    tubules   are    lined,  as  has  been 


vi  THE   HISTORY   OF   UREA  213 

stated,  by  epithelium  cells,  and  these  cells,  in  certain  parts 
of  the  tubule,  especially  where  these  are  coiled,  are  secreting 
cells.  That  is  to  say,  they  have  the  power,  by  some  means 
which  we  do  not  at  present  fully  understand,  to  take  up 
from  the  blood,  which  is  flowing  in  the  capillaries  wound 
round  the  tubules,  or  rather  from  the  plasma  which  exudes 
from  those  capillaries  and  bathes  the  bases  of  the  cells,  cer- 
tain substances,  and  to  pour  these  substances  into  the  cavity 
of  the  tubule. 

And  we  have  evidence  that  many  of  the  most  important 
constituents  of  the  urine,  such  as  urea,  uric  acid,  and  others, 
are  thus  secreted  by  the  epithelium  cells  of  the  tubules,  and 
not  simply  filtered  off  by  the  Malpighian  capsules. 

The  formation  of  urine  is  therefore  a  double  process.  A 
great  deal  of  the  water,  with  probably  some  of  the  more 
soluble  inorganic  salts,  passes  by  the  glomeruli,  but  the 
urea,  the  colouring  matters,  and  a  great  many  other  of  the 
constituents,  are  thrown  into  the  cavities  of  the  tubules  by 
a  peculiar  action  of  the  epithelium  cells. 

7.  The  History  of  Urea.  —  Nitrogen  enters  the  body  as 
proteid  food  and,  practically,  all  of  it  leaves  the  body  again 
as  urea.  Somewhere  or  other,  and  by  some  means  or  other, 
the  nitrogen  while  in  transit  is  turned  over  from  the  proteids 
into  urea.  This  change  involves  the  whole  nitrogenous  me- 
tabolism1 of  the  body  and  from  its  importance  merits  a  short 
statement  of  the  chief  facts  which  throw  some  light  on  the 
question  of  where  and  how  urea  is  formed. 

In  the  first  place  the  urea  excreted  in  the  urine  is  not 
?nade  in  the  kidney  out  of  some  other  (antecedent)  sub- 
stance.    The  activity  of  the  kidney  consists  in  picking  out 

1  The  word  "  metabolism  "  (MeTa/3oA>j  =  change)  is  conveniently  used  to 
denote  the  sum  total  of  those  chemical  changes  which  take  place  in  living 
matter,  and  in  virtue  of  which  we  speak  of  it  as  "  living." 


214  ELEMENTARY   PHYSIOLOGY  less. 

ready-made  urea  from  the  blood  which  passes  through  it 
and  discharging  this  urea  into  the  channels  of  the  tubules. 
Hence  urea  must  be  made  in  tissues  other  than  the  kidney 
and  finds  its  way  from  these  into  the  blood. 

Nearly  half  the  weight  of  the  body  is  made  up  of  muscular 
tissue,  the  muscles.  Even  when  at  rest  these  muscles  are 
the  seat  of  active  oxidation,  and  this  activity  is  enormously 
increased  at  times  when  they  are  contracting.  There  must 
therefore  always  be  a  considerable  wear  and  tear  going  on  in 
them,  and  we  must  suppose  that  this  leads  to  the  formation 
of  waste ;  of  this  some  should  contain  nitrogen,  since  the 
muscles  are  chiefly  built  up  of  nitrogenous  material.  But 
this  waste  does  not  come  out  of  the  muscles  as  ready-made 
urea,  neither  do  we  know  as  yet  exactly  in  what  form  it  does 
leave  them.  In  fact,  all  we  know  is  that  the  muscles  give 
off  nitrogenous  waste,  that  this  waste  is  presumably  turned 
into  urea  in  some  other  part  of  the  body,  and  the  urea 
picked  out  and  excreted  by  the  kidneys. 

The  liver  (p.  233)  is  the  seat  of  many  activities  with  which 
we  shall  deal  later  on,  and  among  these  there  is  no  doubt 
that  the  making  of  urea  out  of  other  substances  brought  to  it 
in  the  blood  is  not  the  least  important  of  them.  We  know 
to  a  certain  extent  what  one  of  these  "  other  substances  "  is. 
When  we  study  digestion  we  shall  see  that  one  of  the 
products  of  digestion  of  proteids  is  a  nitrogenous,  crystalline 
substance  known  as  leucin.  This  is  absorbed  through  the 
walls  of  the  intestines,  carried  to  the  liver  in  the  blood  of 
the  portal  vein,  and  apparently  converted  into  urea  by  the 
liver.  Possibly  the  liver  similarly  converts  other  nitrogenous 
products,  which  it  receives  from  the  tissues,  into  urea.  But 
one  thing  is  certain,  a  considerable  portion  of  the  urea  which 
is  excreted  by  the  kidneys  is  made  in  the  liver.  Beyond  this 
fact  our   knowledge  of  anything  definite  as  to   the  mode 


vi  THE   STRUCTURE   OF  THE   SKIN      ■  215 

of  origin  of  urea  in  the  body  is  very  imperfect  and  incom- 
plete. 

8.  The  Structure  of  the  Skin.  Nails  and  Hairs. — That 
the  skin  is  a  source  of  continual  loss  to  the  blood  may  be 
proved  in  various  ways.  If  the  whole  body  of  a  man,  or  one 
of  his  limbs,  be  inclosed  in  a  rubber  bag,  full  of  air,  it  will 
be  found  that  this  air  undergoes  changes  which  are  similar  in 
kind  to  those  which  take  place  in  the  air  which  is  inspired 
into  the  lungs.  That  is  to  say,  the  air  loses  oxygen  and 
gains  carbonic  acid ;  it  also  receives  a  great  quantity  of 
watery  vapour,  which  condenses  upon  the  sides  of  the  bag, 
and  may  be  drawn  off  by  a  properly  disposed  pipe.  Further, 
there  is  a  continual  loss  of  heat  taking  place  from  the  sur- 
face of  the  body.  Of  these  the  loss  of  watery  vapour  and 
of  heat  are  of  immense  importance,  for  it  is  chiefly  by  means 
of  variations  in  their  amount  from  time  to  time  that  the 
temperature  of  the  body  is  kept  nearly  constant.  But  be- 
fore dealing  with  these  activities  of  the  skin  we  must  under- 
stand the  main  facts  as  to  its  structure. 

The  skin  (Fig.  65)  consists  of  two  parts,  an  outer  layer  or 
epidermis,  resting  on  a  deeper  layer,  the  dermis.  The  skin 
as  a  whole  is  connected  with  the  tissues  it  covers  by  a  layer 
of  loose  fibrous  connective  tissue  (see  Fig.  14),  called  sub- 
cutaneous tissue.  This  often  contains  fat,  and  is  the  part 
which  is  cut  through  when  an  animal  is  skinned. 

The  dermis  is  made  up  of  a  dense  feltwork  of  ordinary 
connective  tissue  fibres  mixed  with  many  elastic  fibres  and 
some  connective  tissue  corpuscles.  The  surface  of  the  der- 
mis is  raised  up  into  little  hillocks  or  elevations  known  as 
the  papillae.  Arteries  enter  the  dermis  and  break  up  into 
capillaries,  which  are  very  close  set  at  its  surface  and  in  the 
papilla?  ;  thus  the  dermis  is  extremely  vascular.  Nerves  also 
run  into  the  dermis,  and  passing  outwards,  form  a  network 


2l6 


ELEMENTARY   PHYSIOLOGY 


of  fibres  at  its  junction  with  the  epidermis,  and  from  this 
network  extremely  fine  nerve  fibrils  pass  out  and  between 


Fig.  65. —  Diagram  to  show  the  Structure  of  the  Skin. 

E.c,  homy  layer  of  epidermis;  E.m,  Malpighian  layer  of  epidermis;  D.c,  con- 
nective tissue  of  dermis;  /,  papilla;  gl,  sweat  gland,  the  coils  of  the  tube  cut  across 
or  lengthwise;   d,  its  duct;_/,  fat;  v,  blood-vessels;  n,  nerve;  t.c,  tactile  corpuscle. 


VI  THE   STRUCTURE   OF   THE    SKIN  217 

the  lower  cells  of  the  epidermis.  In  some  parts  of  the  body, 
some  of  the  branches  of  the  nerves  run  up  into  the  papillae, 
where  they  are  connected  with  special  nervous  structures, 
such  as  tactile  corpuscles  and  end-bulbs.  But  since  these 
are  of  importance  solely  in  connection  with  the  functions  of 
the  skin  as  a  sense-organ,  they  will  be  described  later  on 
(seep.  373). 

The  epidermis  lies  on  the  dermis  and  dips  down  into  all 
Its  depressions.  It  is  composed  entirely  of  cells  and  has 
no  blood-vessels. 

The  cells  may  be  divided  into  two  layers.  Of  these  the 
innermost  or  Malpighian  layer  (Fig.  65,  -E.m)  is  made 
up  of  nucleated  cells,  which  are  tall  and  columnar  where 
they  rest  on  the  dermis,  become  more  rounded  and  wrinkled 
as  they  pass  outwards,  and  then  flattened  and  granular. 
The  outer  layer  of  the  epidermis,  or  horny  layer  (Fig.  65, 
-E.c),  is  made  up  of  cells  which,  losing  their  nuclei,  become 
converted  into  flattened,  thin  scales,  consisting  of  horny 
material.  These  are  the  cells  which  become  so  strongly 
developed  on  parts  of  the  body  subject  to  friction,  such  as 
the  hands  and  soles  of  the  feet.  They  are  always  being 
shed  from  the  surface  of  the  skin,  and  their  place  is  taken 
by  new  cells,  pressed  out  from  the  deeper  layers  of  the 
epidermis   (see  also  pp.  37,  38). 

All  over  the  body  the  skin  presents  minute  apertures, 
the  ends  of  channels  excavated  in  the  epidermis,  and  each 
continuing  the  direction  of  a  minute  tube,  usually  about 
So/a  (^0  of  an  inch)  in  diameter,  and  a  quarter  of  an  inch 
long,  the  end  of  which  is  imbedded  in  the  dermis.  Each 
tube  is  lined  with  an  epithelium  continuous  with  the  epi- 
dermis (Fig.  65,  d).  The  tube  sometimes  divides,  but, 
whether  single  or  branched,  its  inner  end  or  ends  are  blind, 
and  coiled  up  into  a  sort  of  knot,  interlaced  with  a  mesh- 
work  of  capillaries  (Fig.  65,^/,  and  Fig.  66). 


ELEMENTARY   PHYSIOLOGY 


LESS. 


This  coiled-up  portion  is  called  a  sweat-gland,  and  the 
tube  leading  from  it  to  the  surface  of  the  skin  is  its  duct. 
The  cells  lining  the  duct  are  small  and  rounded,  those  in 
the  tube  of  the  gland  are  larger  and  more  columnar,  and 
may  be  readily  stained. 

The  blood  in  the  capillaries  of  the  gland  is  separated 
from  the  cavity  of  the  sweat-gland  only  by  the  thin  walls 


Fig.  66. —  A  Sweat-gland  (Fig.  65,^/),  Epithelium  not  shown. 
a,  the  gland;  b,  the  duct;  c,  network  of  capillaries,  inside  which  the  gland  lies. 


of  the  capillaries  and  the  glandular  epithelium,  which 
together  constitute  but  a  very  thin  pellicle.  This  arrange- 
ment, though  different  in  detail  from,  is  similar  in  principle 
to,  that  which  obtains  in  the  kidney.  In  the  latter,  the 
vessel  makes  a  coil  within  the  Malpighian  capsule,  which 
ends  a  uriniferous  tubule.  Here  the  perspiratory  tubule 
coils  about  and  among  the  vessels.  In  both  cases  the  same 
result  is  arrived  at  —  namely,  the  exposure  of  the  blood  to  a 


vi  THE   STRUCTURE   OF  THE   SKIN  219 

large,  relatively  free  surface,  upon  which  certain  of  its  con- 
tents transude.  In  the  sweat-gland,  however,  there  is  no 
filtering  apparatus  like  the  Malpighian  corpuscle  of  the 
kidney,  and  the  whole  of  the  sweat  appears  to  be  secreted 
into  the  interior  of  the  tube  by  the  action  of  the  epithelium 
cells  which  line  it. 

The  number  of  these  glands  varies  in  different  parts  of  the 
body.  They  are  fewest  in  the  back  and  neck,  where  their 
number  is  not  much  more  than  400  to  a  square  inch. 
They  are  more  numerous  on  the  skin  of  the  palm  and 
sole,  where  their  apertures  follow  the  ridges  visible  on  the 
skin,  and  amount  to  between  two  and  three  thousand  on 
the  square  inch.  At  a  rough  estimate,  the  whole  integu- 
ment probably  possesses  not  fewer  than  from  two  millions 
and  a  quarter  to  two  millions  and  a  half  of  these  tubules, 
which  therefore  must  possess  a  very  great  aggregate  secret- 
ing power. 

In  certain  regions  of  the  skin  the  horny  cells  of  the 
epidermis  are  not  at  once  thrown  off  in  flakes,  but  are  at 
first  built  up  in  definite  structures  known  as  nails  and  hairs, 
which  grow  by  constant  addition  to  the  surfaces  by  which 
they  adhere  to  the  epidermis.  In  the  case  of  the  nails  the 
process  of  growth  has  no  limit,  and  the  nail  is  kept  of  one 
size  simply  by  the  wearing  or  cutting  away  of  its  oldest 
or  free  end.  In  the  case  of  the  hairs,  on  the  contrary, 
the  growth  of  each  hair  is  limited,  and  when  its  term  is 
reached  the  hair  falls  out  and  is  replaced  by  a  new  hair. 

Underneath  each  nail  the  deep  or  dermal  layer  of  the 
integument  is  peculiarly  modified  to  form  the  bed  of  the 
nail.  It  is  very  vascular,  and  raised  up  into  numerous 
parallel  ridges,  like  elongated  papillae  (Fig.  67,  B,  C).  The 
surfaces  of  all  these  are  covered  with  growing  epidermic 
cells,  which,   as   they  flatten  and   become    converted    into 


22C  ELEMENTARY   PHYSIOLOGY  less. 

horn,  form  a  solid  continuous  plate,  the  nail.  At  the  hinder 
part  of  the  bed  of  the  nail  the  integument  forms  a  deep 
fold,  from  the  bottom  of  which,  in  like  manner,  new  epider- 


FlG.    67. 

A,  a  longitudinal  and  vertical  section  of  a  nail;  a,  the  fold  at  the  base  of  the 
nail:  b,  the  nail;  c,  the  bed  of  the  nail.  The  figure  B  is  a  transverse  section  of  the 
same  —  a,  a  small  lateral  fold  of  the  integument;  i,  nail;  c,  bed  of  the  nail,  with  its 
ridges.  The  figure  C  is  a  highly-magnified  view  of  a  part  of  the  foregoing  —  c,  the 
ridges;  d,  the  deep  layers  of  epidermis;  e,  the  horny  scales  coalesced  into  nail  sub- 
stance. (Figs.  A  and  B  magnified  about  4  diameters;  Fig.  C  magnified  about  200 
diameters.) 

mal  cells  are  added  to  the  base  of  the  nail,  which  is  thus 
constrained  to  move  forward. 

The  nail  thus  constantly  receiving  additions  from  below 


THE   STRUCTURE  OF  THE   SKIN 


and  from  behind,  slides  forwards  over  its  bed,  and  projects 
beyond  the  end  of  the  finger,  where  it  is  worn  away  or 
cut  off. 


Fig.  68. — A  Hair  in  its  Hair-sac. 

a,  shaft  of  hair  above  the  skin;  b,  cortical  substance  of  the  shaft,  the  medulla  not 
being  visible;  c,  newest  portion  of  hair  growing  on  the  papilla  (/)  ;  d,  cuticle  of  hair; 
f,  cavity  of  hair-sac;  f,  epidermis  (and  root-sheaths)  of  the  hair-sac,  corresponding  to 
the  Malpighian  layer  of  the  epidermis  of  the  integument  («/):  g>  division  between 
dermis  and  epidermis;  //,  dermis  of  hair-sac  corresponding  to  dermis  of  integument  (/); 
k,  mouths  of  sebaceous  glands;  n,  horny  layer  of  epidermis  of  integument. 

A  hair,  like  a  nail,  is  composed  of  horny  cells ;  but 
instead  of  being  only  partially  sunk  in  a  fold  of  the  integu- 


222  ELEMENTARY    PHYSIOLOGY  less. 

ment  it  is  at  first  wholly  inclosed  in  a  kind  of  bag,  the  hair- 
sac  or  follicle,  from  the  bottom  of  which  a  papilla  (Fig. 
68,  i),  which  answers  to  a  single  ridge  of  the  nail,  arises. 
The  hair  is  developed  by  the  conversion  into  horn,  and 
coalescence  into  a  shaft,  of  the  superficial  epidermal  cells 
coating  the  papilla.  These  coalesced  and  cornified  cells 
being  continually  replaced  by  new  growths  from  below, 
which  undergo  the  same  metamorphosis,  the  shaft  of  the 
hair  is  thrust  out  until  it  attains  the  full  length  natural  to  it. 
Its  base  then  ceases  to  grow,  and  the  old  papilla  and  sac 
die  away,  but  not  before  a  new  sac  and  papilla  have  been 


dc  £ 


Fig.  69. — Part  of  the  Shaft  of  a  Hair  inclosed  within  its  Root-sheaths 
and  treated  with  Caustic  Soda,  which  has  caused  the  Shaft  to  be- 
come distorted. 

a,   medulla;    b,  cortical  substance;   c,  cuticle  of  the  shaft;  from  d  to  f,  the  root- 
sheaths,  in  section.      (Magnified  about  200  diameters.) 


formed  by  budding  from  the  sides  of  the  old  one.  These 
give  rise  to  a  new  hair.  The  shaft  of  a  hair  of  the  head 
consists  of  a  central  pith  or  medullary  matter  (Fig.  69,  a), 
of  a  loose  and  open  texture,  which  sometimes  contains  air 
and  is  often  wanting  altogether ;  of  a  cortical  or  fibrous 
substance  (Fig.  69,  l>),  surrounding  this,  made  up  of 
coalesced  elongated  horny  cells  and  containing  pigment ; 
and  of  an  outer  cuticle  (Fig.  69,  c)  composed  of  flat  horny 
plates,  arranged  transversely  round  the  shaft,  so  as  to  over- 
lap one  another   by  their   outer   edges,    like    tiles   on   the 


\ri      THE   COMPOSITION  AND   QUANTITY   OF   SWEAT     223 

roof  of  a  house.  The  superficial  epidermal  cells  of  the 
hair-sac  also  coalesce  by  their  edges,  and  become  converted 
into  root-sheaths  (Fig.  69,  d,  £,f), which  embrace  the  root 
of  the  hair,  and  usually  come  away  with  it  when  it  is 
plucked  out. 


Fig.  70.  — Section  of  the  Skin,  showing  the  Roots  of  the  Hairs  and 
the  Sebaceous  Glands. 

a,  epidermis;  b,  muscle  of  c  the  hair-sheath,  on  the  left  hand;  d,  dermis;  e,  twoseba- 
ceous  glands  attached  to  each  hair-sac. 

The  sebaceous  glands  (Fig.  70)  are  small  glands  whose 
duct  opens  into  the  follicle  of  a  hair.  They  form  a  fatty 
secretion  which  lubricates  the  hairs. 

9.  The  Composition  and  Quantity  of  Sweat.  —  The 
sweat-glands  have  the  function  of  forming  a  fluid,  the 
sweat,  which  is  passed  out  upon  the  surface  of  the  body. 
This  fluid  is  composed  chiefly  of  water  containing  a  small 
amount  (1-2  per  cent.)  of  solid  matter  in  solution,  of  which 
sodium  chloride  is  a  prominent  constituent.  In  health, 
sweat  contains  no  appreciable  amount  of  urea. 

In  its  normal  state  the  sweat,  as  poured  out  from  the 
proper  sweat-glands,  is  alkaline  ;  but  ordinarily,  as  it  col- 
lects upon  the  skin,  it  is  mixed  with  the  fatty  secretion  of 
the  sebaceous  glands,  and  then  is  frequently  acid.  In  addi- 
tion it  contains  scales  of  the  external  layers  of  the  epidermis, 
which  are  constantly  being  shed. 

Under  ordinary  conditions  the  sweat  is  evaporated  from 
the  surface  of  the  skin  as  fast  as  it  is  secreted  \  in  this  case  it 


224  ELEMENTARY   PHYSIOLOGY  less. 

is  frequently  spoken  of  as  insensible  perspiration.  But  when 
violent  exercise  is  taken,  or  when  under  some  kind  of  mental 
emotion,  or  when  the  body  is  exposed  to  a  hot  and  moist 
atmosphere,  the  sweat  is  secreted  faster  than  it  evaporates : 
the  perspiration  then  becomes  sensible,  that  is,  it  appears  in 
the  form  of  scattered  drops  on  the  surface  of  the  body. 

The  quantity  of  sweat,  or  sensible  perspiration,  and  also 
the  total  amount  of  both  sensible  and  insensible  perspiration, 
vary  immensely,  according  to  the  temperature  and  other  con- 
ditions of  the  air,  and  according  to  the  state  of  the  blood 
and  of  the  nervous  system.  It  is  estimated  that,  as  a  gen- 
eral rule,  the  quantity  of  water  excreted  by  the  skin  is  con- 
siderably more  than  that  given  out  by  the  lungs  in  the  same 
time. 

The  amount  of  matter  which  may  be  lost  by  perspiration 
under  certain  circumstances,  is  very  remarkable.  Heat  and 
severe  labour,  combined,  may  reduce  the  weight  of  a  man 
two  or  three  pounds  in  an  hour,  by  means  of  the  cutaneous 
perspiration  alone  ;  and,  as  there  is  some  reason  to  believe 
that  the  quantity  of  solid  matter  carried  off  from  the  blood 
does  not  diminish  with  the  increase  of  the  amount  of  the 
perspiration,  the  total  amount  of  solids  which  are  eliminated 
by  profuse  sweating  may  be  considerable. 

10.  The  Secretion  of  Sweat  and  its  Nervous  Control.  — 
In  analysing  the  process  by  which  the  perspiration  is  elimi- 
nated from  the  body,  it  must  be  recollected,  in  the  first 
place,  that  the  skin,  even  if  there  were  no  glandular  struc- 
tures connected  with  it,  would  be  in  the  position  of  a  mod- 
erately thick,  permeable  membrane,  interposed  between  a  hot 
fluid,  the  blood,  and  the  atmosphere.  Even  in  hot  climates 
the  air  is,  usually,  far  from  being  completely  saturated  with 
watery  vapour,  and  in  temperate  climates  it  ceases  to  be  so 
saturated  the  moment  it  comes  into  contact  with  the  skin, 


vi  THE   SECRETION    OF   SWEAT  225 

the  temperature  of  which  is,  ordinarily,  twenty  or  thirty 
degrees  above  its  own. 

A  bladder  exhibits  no  sensible  pores ;  but  if  a  bladder  be 
filled  with  water  and  suspended  in  the  air,  the  water  will 
gradually  ooze  through  the  walls  of  the  bladder,  and  disap- 
pear by  evaporation.  Now,  in  its  relation  to  the  blood,  the 
skin  is  such  a  bladder  full  of  hot  fluid. 

Thus,  perspiration  to  a  certain  amount  must  always  be 
going  on  through  the  substance  of  the  integument,  but  prob- 
ably not  to  any  great  extent ;  though  what  the  amount  of 
this  perspiration  may  be  cannot  be  accurately  ascertained, 
because  it  is  entirely  masked  by  the  secretion  from  the 
sweat-glands. 

When  from  any  ordinary  cause  an  increased  formation  of 
sweat  takes  place,  two  things  usually  happen.  The  small 
arteries  which  supply  the  capillary  network  surrounding  the 
coiled  tube  of  the  sweat-gland  dilate  and  there  is  an  increased 
flow  of  blood  through  these  capillaries.  At  the  same  time 
the  cells  of  the  glands  begin  to  pour  out  an  increased  quan- 
tity of  fluid,  in  other  words  they  begin  to  secrete.  The 
first  of  the  above  two  results  is  brought  about  by  a  lessening 
of  the  vaso-constrictor  impulses  which  had  previously  been 
keeping  the  arteries  constricted  (see  p.  94).  But  what,  on 
the  other  hand,  is  the  cause  of  the  simultaneously  increased 
activity  of  the  sweat-glands?  Do  they  simply  secrete  faster 
because  of  the  increased  supply  of  blood  brought  to  them, 
as  is  the  case  with  the  Malpighian  capsules  of  the  kidney?  Or 
is  it  because  their  cells  are  urged  on  to  greater  activity  by 
special  nervous  impulses  sent  to  them  ?  The  latter  is  the 
real  explanation  of  the  increased  activity  of  the  sweat-cells, 
as  is  shown  by  the  following  facts. 

It  is  possible  to  obtain  an  increased  secretion  of  sweat  by 
the  stimulation  of  nerves  in  parts  of  an  animal's  body  from 
9 


226  ELEMENTARY   PHYSIOLOGY  less. 

which  the  blood-supply  has  been  previously  cut  off.  Again, 
certain  drugs  may  lead  to  sweating  without  at  the  same  time 
producing  any  vascular  changes,  and  the  same  effect  is  often 
observed  in  sweating  which  results  from  mental  emotions 
and  in  the  "  cold  sweats  "  of  a  disease  such  as  phthisis.  The 
nerves  which  can  thus  make  the  cells  of  the  sweat-glands 
become  more  active  may  be  called  secretory  nerves.  They 
appear  to  be  connected  with  a  centre  or  centres  in  the  cen- 
tral nervous  system,  the  number  and  exact  location  of  which 
are  not  fully  known,  and  by  this  means  sweating  may  be 
brought  about  reflexly,  as  when  placing  mustard  in  the  mouth 
causes  the  face  to  sweat.  The  possibility  of  such  reflex 
stimulation  of  the  sweat-glands  acquires  an  extraordinary 
importance,  as  we  shall  see  when  we  come  to  consider  the 
means  by  which  the  temperature  of  the  body  is  regulated 
(p.   231). 

The  ideas  we  have  thus  arrived  at  as  to  the  process  of 
sweat  secretion  hold  good  for  all  secreting  glands ;  and  we 
shall  have  to  consider  them  again  later  on,  when  dealing 
with  certain  of  the  salivary  glands,  in  which  this  indepen- 
dence of  secretion  and  blood-supply  is  much  more  strikingly 
shown  (see  Lesson  VII.). 

11.  A  Comparison  of  the  Lungs,  Kidneys,  and  Skin. — 
It  will  now  be  instructive  to  compare  together  in  more  detail 
than  has  been  done  in  the  first  Lesson  (p.  23)  the  three 
great  organs  —  lungs,  kidneys,  and  skin  —  which  have  been 
described. 

In  ultimate  anatomical  analysis,  each  of  these  organs  con- 
sists of  a  moist  animal  membrane  separating  the  blood  from 
the  atmosphere. 

Water,  carbonic  acid,  and  solid  matter  pass  out  from  the 
blood  through  the  animal  membrane  in  each  organ,  and  con- 
stitute its  secretion  or  excretion  ;  but  the  three  organs  differ 


vi  ANIMAL    HEAT  22? 

in  the  absolute  and  relative  amounts  of  the  constituents  the 
escape  of  which  they  permit. 

Taken  by  weight,  water  is  the  predominant  excretion  in 
all  three  ;  most  solid  matter  is  given  off  by  the  kidneys ; 
most  gaseous  matter  by  the  lungs. 

The  skin  partakes  of  the  nature  of  both'  lungs  and  kidneys, 
seeing  that  it  absorbs  oxygen  and  exhales  carbonic  acid  and 
water,  like  the  former,  while  it  excretes  organic  and  saline 
matter  in  solution,  like  the  latter ;  but  the  skin  is  more 
closely  related  to  the  kidneys  than  to  the  lungs.  Hence, 
as  has  been  already  said,  when  the  free  action  of  the  skin  is 
interrupted,  its  work  is  usually  thrown  upon  the  kidneys,  and 
vice  versa.  In  hot  weather,  when  the  excretion  by  the  skin 
increases,  that  of  the  kidneys  diminishes,  and  the  reverse  is 
observed  in  cold  weather. 

This  power  of  mutual  substitution,  however,  only  goes  a 
little  way ;  for  if  the  kidneys  be  extirpated,  or  their  func- 
tions much  interfered  with,  death  ensues,  however  active  the 
skin  may  be.  And,  on  the  other  hand,  if  the  skin  be  cov- 
ered with  an  impenetrable  varnish,  the  temperature  of  the 
body  rapidly  falls,  and  from  this  cause  death  takes  place, 
though  the  lungs  and  kidneys  remain  active. 

12.  Animal  Heat :  its  Production  and  Distribution.  —  It 
has  been  seen  that  heat  is  being  constantly  given  off  from 
the  skin  and  from  the  air-passages ;  and  everything  that 
passes  from  the  body  carries  away  with  it,  in  like  manner,  a 
certain  quantity  of  heat.  Furthermore,  the  surface  of  the 
body  is  much  more  exposed  to  cold  than  its  interior.  Nev- 
ertheless, the  temperature  of  the  body  is  in  health  main- 
tained very  evenly,  at  all  times  and  in  all  parts,  within  the 
range  of  two  degrees  or  even  less  on  either  side  of  370  C. 
(98.60  F.). 

This    is   the    result    of  three  conditions :    the  first,  that 


228  ELEMENTARY   PHYSIOLOGY  less. 

heat  is  constantly  being  generated  in  the  body ;  the  second, 
that  it  is  as  constantly  being  distributed  through  the  body ; 
the  third,  that  it  is  subject  to  incessant  regulation  as  regards 
both  loss  and  production. 

Heat  is  generated  whenever  oxidation  takes  place.  As 
we  have  seen,  the  tissues  all  over  the  body,  muscles,  brain- 
substance,  gland  cells,  and  the  like,  are  continually  under- 
going oxidation.  The  living  substance  of  the  tissue,  built 
up  out  of  the  complex  proteids,  fats,  and  carbohydrates,  and 
thus  even  still  more  complex  than  these,  is,  by  means  of 
the  oxygen  brought  by  the  arterial  blood,  oxidised,  and 
broken  down  into  simpler,  more  oxidised  bodies,  which  are 
eventually  reduced  to  urea,  carbonic  acid,  and  water.  Wher- 
ever life  is  being  manifested  these  oxidative  changes  are 
going  on,  more  energetically  in  some  places,  in  some  tis- 
sues, and  in  some  organs,  than  in  others.  Hence  every 
capillary  vessel  and  every  extra-vascular  islet  of  tissue  is 
really  a  small  fireplace  in  which  heat  is  being  evolved,  in 
proportion  to  the  activity  of  the  chemical  changes  which 
are  going  on. 

The  chief  seat  of  this  heat  production  is  undoubtedly  in 
the  muscles  ;  for,  as  already  pointed  out,  they  make  up 
about  half  the  body-weight,  and  are  carrying  on  an  active 
oxidation  even  while  at  rest.  This  gives  rise  to  heat,  and 
when  a  muscle  enters  into  a  state  of  contracting  activity, 
the  heat  production  becomes  so  rapid  as  to  produce  an 
actual  measurable  rise  of  its  temperature.  After  the  mus- 
cles we  may  regard  the  liver  as  the  next  great  heat-produc- 
ing organ  of  the  body. 

But  as  the  vital  activities  of  different  parts  of  the  body, 
and  of  the  whole  body,  at  different  times,  are  very  different  I 
and  as  some  parts  of  the  body  are  so  situated  as  to  lose  their 
heat  by  radiation  and  conduction  much  more  easily  than 


vi  REGULATION   OF  BODY-TEMPERATURE  229 

others,  the  temperature  of  the  body  would  be  very  unequal 
in  its  different  parts,  and  at  different  times,  were  it  not  for 
the  arrangement  by  which  the  heat  is  distributed  and  regu- 
lated. 

Whatever  oxidation  occurs  in  any  part,  raises  the  tempera- 
ture of  the  blood  which  is  in  that  part  at  the  time,  to  a  pro- 
portional extent.  But  this  blood  is  swiftly  hurried  away  into 
other  regions  of  the  body,  and  rapidly  gives  up  its  excess 
heat  to  them.  On  the  other  hand,  the  blood  which,  by 
being  carried  to  the  vessels  in  the  skin  on  the  surface  of  the 
body,  begins  to  have  its  temperature  lowered  by  evaporation, 
radiation,  and  conduction,  is  hurried  away,  before  it  has 
time  to  get  thoroughly  cooled,  into  the  deeper  organs ;  and 
in  them  it  becomes  warm  by  contact,  as  well  as  by  the  oxi- 
dating processes  there  going  on.  Thus  the  blood-vessels 
and  their  contents  may  be  compared  to  a  system  of  hot- 
water  pipes,  through  which  the  warm  water  is  kept  con- 
stantly circulating  by  a  pump ;  while  it  is  heated,  not  by  a 
great  central  boiler  as  usual,  but  by  a  multitude  of  minute 
gas  jets,  disposed  beneath  the  pipes,  not  evenly,  but  more 
here  and  fewer  there.  It  is  obvious  that,  however  much 
greater  might  be  the  heat  applied  to  one  part  of  the  system 
of  pipes  than  to  another,  the  general  temperature  of  the 
water  would  be  even  throughout,  if  it  were  kept  moving 
with  sufficient  quickness  by  the  pump.  In  this  way,  then, 
the  temperature  of  the  body  is  kept  uniform  in  its  several 
parts. 

13.  Regulation  of  Body-temperature  by  Altered  Loss  of 
Heat.  —  If  a  system  such  as  we  have  just  imagined  were 
entirely  composed  of  closed  pipes,  the  temperature  of  the 
water  might  be  raised  to  any  extent  by  the  gas  jets.  On  the 
other  hand,  it  might  be  kept  down  to  any  required  degree 
by  causing  a  larger,  or  smaller,  portion  of  the  pipes  to  be 


23o  ELEMENTARY   PHYSIOLOGY  less. 

wetted  with  water,  which  should  be  able  to  evaporate  freely 
—  as,  for  example,  by  wrapping  them  in  wet  cloths.  And 
the  greater  the  quantity  of  water  thus*  evaporated,  the  lower 
would  be  the  temperature  of  the  whole  apparatus. 

Now,  the  regulation  of  the  temperature  of  the  human  body 
is  chiefly  effected  on  this  principle.  The  vessels  are  closed 
pipes,  but  a  great  number  of  them  are  inclosed  in  the  skin 
and  in  the  mucous  membrane  of  the  air-passages,  which  are, 
in  a  physical  sense,  wet  cloths  freely  exposed  to  the  air.  It 
is  the  evaporation  from  these  which  exercises  a  more  im- 
portant influence  than  any  other  condition  upon  the  regula- 
tion of  the  temperature  of  the  blood,  and,  consequently,  of 
the  body. 

But,  as  a  further  nicety  of  adjustment,  the  wetness  of  the 
regulator  is  itself  determined,  through  the  aid  of  the  nervous 
system,  by  the  temperature  of  the  body.  The  sweat-glands, 
as  we  have  seen,  may  be  made  to  secrete  by  impulses  reach- 
ing them  along  certain  nerves  coming  from  a  centre,  or 
centres,  in  the  central  nervous  system.  This  centre  is  itself 
connected  by  other  nerves  with  the  skin,  and  the  ends  of 
these  cutaneous  nerves  are  so  constituted  that  they  are 
stimulated  by  heat  applied  to  the  skin.  When  the  body  is 
exposed  to  a  high  temperature  (and  the  same  occurs  when 
a  part  only  of  the  body  is  heated),  these  cutaneous  nerves 
convey  impulses  to  the  central  nervous  system,  from  which 
other  impulses  are  then  sent  out  along  the  secretory  nerves 
to  the  sweat-glands  and  cause  them  to  pour  forth  a  copious 
secretion  on  to  the  skin ;  and  when  the  temperature  falls, 
the  glands  cease  to  act.  Moreover,  in  this  work  of  secret- 
ing sweat,  the  sweat-glands  are  assisted  by  corresponding 
changes  in  the  blood-vessels  of  the  skin.  It  has  been  stated 
(see  p.  91)  that  the  small  arteries  of  the  body  may  be  some- 
times narrowed  or  constricted,  and  sometimes  widened  or 


vi  REGULATION   OF   BODY-TEMPERATURE  231 

dilated.  Now  the  condition  of  the  small  arteries,  whether 
they  are  constricted  or  dilated,  depends,  as  we  have  also 
seen,  upon  the  action  of  certain  nerves  (vaso-motor  nerves). 
And  it  appears  that  when  the  body  is  exposed  to  a  high 
temperature  these  nerves  are  so  affected  as  to  lead  to  a 
dilation  of  small  arteries  of  the  skin  ;  but  when  these  are 
dilated  the  capillaries  and  small  veins  in  which  they  end 
become  much  fuller  of  blood,  and  from  these  filled  and 
swollen  capillaries  much  more  nutritive  matter  passes  through 
the  capillary  walls  to  the  sweat-glands,  so  that  these  have 
more  abundant  material  from  which  to  manufacture  sweat. 
On  the  other  hand,  when  the  body  is  lowered  in  tempera- 
ture the  vaso-motor  nerves  are  so  affected  that  the  small 
arteries  of  the  skin  are  constricted ;  hence  less  blood  enters 
the  capillaries  of  the  skin,  and  less  material  is  brought  to 
the  sweat-glands. 

Thus  when  the  temperature  is  raised  two  things  happen, 
both  brought  about  by  the  nervous  system.  In  the  first 
place,  the  arteries  of  the  skin  are  widened  so  that  a  much 
larger  proportion  of  the  total  blood  of  the  body  is  carried 
to  the  surface  of  the  skin  and  there  becomes  cooled ;  and, 
secondly,  this  cooling  process  is  greatly  helped  by  the  in- 
creased evaporation  resulting  from  the  increased  action  of 
the  sweat-glands,  whose  activity  is  further  favoured  by  the 
presence  in  the  skin  of  so  much  blood.  Conversely,  when 
the  temperature  is  lowered,  less  of  the  blood  is  brought  to 
the  skin,  and  more  of  the  blood  circulates  through  the 
deeper,  hotter  parts  of  the  body,  and  the  sweat-glands  cease 
their  work  (this  quiescence  of  theirs  being  in  turn  favoured 
by  the  lessened  blood-supply)  ;  hence  the  evaporation  is 
largely  diminished,  and  thus  the  blood  is  much  less  cooled. 

Hence  it  is  that,  so  long  as  the  surface  of  the  body  per- 
spires freely,  and  the  air-passages  are  abundantly  moist,  a 


232  ELEMENTARY   PHYSIOLOGY  less. 

man  may  remain  with  impunity,  for  a  considerable  time, 
in  an  oven  in  which  meat  is  being  cooked.  The  heat  of 
the  air  is  expended  in  converting  this  superabundant  per- 
spiration into  vapour,  and  the  temperature  of  the  man's 
blood  is  hardly  raised. 

14.  Regulation  of  Body-temperature  by  Altered  Pro- 
duction of  Heat. — The  temperature  of  the  body  is  kept 
constant  by  that  carefully  adjusted  variation  in  loss  of  heat 
from  its  surface  which  has  been  described  in  the  preceding 
section.  But  now  we  may  point  out  that  there  is  another 
way  by  which  this  constancy  might  be  attained,  namely,  by 
altering  the  production  of  heat  taking  place  in  the  body,  in 
correspondence  to  the  changes  of  the  surrounding  tempera- 
ture ;  just  as  the  temperature  of  a  room  may  be  regulated 
by  putting  out  or  increasing  the  fire  as  well  as  by  opening 
or  closing  its  windows.  The  question  thus  raised  is  very 
interesting,  but  it  is  also  very  abstruse,  and  we  must  not  do 
more  than  just  touch  upon  it. 

All  oxidation  in  the  body  involves  the  consumption  of 
oxygen,  the  production  of  carbonic  acid  and  the  genera- 
tion of  an  exactly  corresponding  quantity  of  heat.  We  may, 
therefore,  take  the  difference  in  the  amount  of  oxygen  used 
up  (and  of  carbonic  acid  produced)  at  different  times  as 
a  measure  of  the  amount  of  heat  produced  in  the  body 
during  the  same  periods.  Working  in  this  way  it  is  found 
that  when  a  warm-blooded  animal  is  exposed  to  cold,  as 
when  it  is  put  into  a  chamber  which  is  cooled,  it  uses  up 
more  oxygen  and  gives  off  more  carbonic  acid  than  when 
put  into  a  warm  chamber.  But  this  can  only  mean  that  in 
the  cooler  surroundings  the  animal  makes  more  heat  than 
when  the  surroundings  are  warm.  Again  we  may  point  out, 
as  tending  to  the  same  conclusions,  that  our  desire  for  food 
is  greater,  on  the  whole,  in  the  cooler  winter  time  than  in 


vi  THE   STRUCTURE   OF  THE   LIVER  233 

the  warmer  summer ;  and  all  food  is  oxidised  in  the  body, 
and  during  this  oxidation  gives  rise  to  heat.  Thus,  there 
are  reasons  for  supposing  that  within  certain  limits  altered 
production  of  heat  may  play  some  part  in  keeping  the 
temperature  of  the  body  constant. 

All  the  functions  of  the  body  which  we  have  so  far  studied 
have  been  seen  to  be  under  the  guidance  of  the  nervous 
system.  We  may,  therefore,  suppose  that  the  production  of 
heat  will  be  no  exception  to  the  rule,  and,  indeed,  there  are 
reasons,  based  largely  on  experiment  and  partly  on  the  phe- 
nomena of  certain  diseases,  which  justify  this  view.  But  the 
nervous  mechanism  of  this  function  is  not  yet  fully  known. 

15.  The  Temperature  of  Fever. — The  condition  to 
which  the  name  of  fever  is  given  is  characterised  essentially 
by  the  temperature  of  the  body  being  higher  than  is  usual 
in  health.  Thus  it  may  rise  to  as  much  as  410  C.  (105 .8°  F.), 
or  occasionally  even  above  this  point,  and  there  has  been 
much  dispute  as  to  how  this  high  temperature  arises.  A 
common  cause  is  a  disturbance  of  the  mechanism  by  which 
heat  is  lost  to  the  body,  some  diminution  in  loss  of  heat 
leading  naturally  to  a  rise  of  temperature.  On  the  other 
hand,  direct  measurement  shows  that  a  fevered  person  often 
gives  off  more  heat  than  usual  and  at  the  same  time  uses  up 
more  oxygen  and  produces  more  carbonic  acid  and  urea 
than  usual.  In  such  cases  there  is  no  doubt  that  the 
abnormally  high  temperature  is  largely  due  to  an  over- 
production of  heat. 

16.  The  Structure  of  the  Liver. — The  liver  is  a  con- 
stant source  both  of.  loss,  and,  in  a  sense,  of  gain,  to  the 
blood  which  passes  through  it.  It  gives  rise  to  loss,  because 
it  secretes  a  peculiar  fluid,  the  bile,  from  the  blood,  and 
throws  that  fluid  into  the  intestine.  It  is  also  in  another 
way  a  source  of  loss  because  it  elaborates  from  the  blood 


234  ELEMENTARY   PHYSIOLOGY  less. 

passing  through  it  a  substance  called  glycogen,  which  is 
stored  up  sometimes  in  large,  sometimes  in  small,  quantities 
in  the  cells  of  the  liver.  This  latter  loss,  however,  is  only 
temporary,  and  may  be  sooner  or  later  converted  into  a 
gain,  for  this  glycogen  very  readily  passes  into  sugar,  and 
either  in  that  form  or  in  some  other  way  is  carried  off  by 
the  blood.  In  this  respect,  therefore,  there  is  a  gain  to 
the  blood  of  kind  or  quality,  though  not  of  quantity  of 
material 


Fig.  71.  —  The  Liver  of  a  Young  Subject  sketched  from  below  and 
behind.     (From  Moore's  Elementary  Physiology.} 

R.L.   right  lobe;    L.L.  left  lobe;  g.bl.  gall-bladder;  v.c.i.  inferior  vena  cava; 
/.  portal  vein;    on  its  right  the  bile-duct,  on  its  left  the  hepatic  artery. 

The  liver  is  the  largest  glandular  organ  in  the  body,  ordi- 
narily weighing  about  1,400-1,700  grammes  (fifty  or  sixty 
ounces).  It  is  a  broad,  dark,  red-coloured  organ,  which 
lies  on  the  right  side  of  the  body,  immediately  below  the 
diaphragm,  with  which  its  upper  surface  is  in  contact, 
while  its  lower  surface  touches  the  intestines  and  the 
right  kidney. 

The    liver   is    invested   by  a  coat  of  peritoneum,  which 


vi  THE   STRUCTURE   OF  THE   LIVER  235 

keeps  it  in  place.  It  is  flattened  from  above  downwards 
and  convex  and  smooth  above,  where  it  fits  into  the  con- 
cavity of  the  lower  surface  of  the  diaphragm  (Fig.  71).  It 
is  concave  and  irregular  below,  where  it  is  in  contact  with 
the  stomach,  the  intestine,  and  the  right  kidney,  irregular 
behind,  and  ends  in  a  thin  edge  in  front. 

Viewed  from  behind  and  below,  as  in  Fig.  71,  the  inferior 
vena  cava,  v.c.i.,  is  seen  to  traverse  a  notch  in  the  hinder 
edge  of  the  liver  as  it  passes  from  the  abdomen  to  the 
thorax.  At/  the  trunk  of  the  portal  vein  is  observed  enter- 
ing into  the  substance  of  the  organ.  At  its  left  the  hepatic 
artery,  coming  almost  directly  from  the  aorta,  similarly 
enters  the  liver,  and  ramifies  through  it.  At  the  right  of 
the  portal  vein  is  the  single  trunk  of  the  duct  called  the 
hepatic  duct,  which  conveys  away  to  the  intestine  the  bile 
brought  to  it  by  its  right  and  left  branches  from  the  liver. 
Opening  into  the  hepatic  duct  is  seen  the  duct  of  a  large 
oval  sac,  g.bl.,  the  gall-bladder. 

The  liver  consists  of  two  chief  lobes,  of  which  the  right 
is  much  larger  than  the  left.  Externally  the  lobes  are 
covered  with  a  layer  of  connective  tissue  forming  its  capsule, 
and  a  quantity  of  connective  tissue  forms  a  thick  sheath 
for  the  portal  vein,  the  hepatic  artery,  and  the  bile-duct 
as  these  plunge  into  the  liver.  This  sheath  accompanies 
the  vessels  as  they  ramify  in  the  liver,  and  finally  forms  a 
number  of  partitions,  continuous  with  the  capsule  on  the 
outside,  which  divide  each  lobe  into  a  very  large  number 
of  small  divisions  called  lobules  (Figs.  72,  A,  and  73,  L). 
These  partitions  are  much  thicker  and  more  conspicuous  in 
some  animals,  such  as  the  pig,  than  they  are  in  others,  such 
as  the  rabbit ;  in  the  former  it  is  very  easy  to  see  on  the 
outside  of  the  liver  the  outlines  of  the  lobules;  in  the  latter 
it  is  not  so  easy.      The  lobules  are  polyhedral  in  shape  and 


236 


ELEMENTARY   PHYSIOLOGY 


LESS, 


about  y1^-  of  an  inch  in  diameter,  being  thus  visible  to  the 
naked  eye.  Each  lobule  is  seated  on  the  branch  of  the 
hepatic  vein,  the  large  vein  which  carries  the  blood  away 
from  the  liver,  and  is  made  up  of  a  mass  of  cells,  the  hepatic 


(From  Quain's  Anatomy.) 


A.  Two  lobules  of  the  liver  (diagrammatic)  (Schafer).  /,  interlobular  branches 
o.  the  portal  vein,  giving  off  capillaries  into  the  lobules:  //,  intralobular  veins,  shown 
in  cross-section  in  the  left-hand  lobule,  in  longitudinal  section  in  the  right-hand 
lobule;  s,  sublobular  branch  nf  the  hepatic  vein:  the  arrows  indicate  the  direction  o( 
the  course  of  the  blood.  The  liver  cells  are  represented  in  a  portion  only  of  each 
lobule. 

B.  Portion  of  lobule  very  highly  magnified,  a,  liver  cell  with  n,  nucleus  (two 
are  often  present):  b,  capillaries  cut  across;  c,  minute  biliary  passages  between  the 
celfs,  injected  with  colouring  matter. 


THE   STRUCTURE   OF  THE   LIVER 


237 


cells,  which  lie  in  the  meshes  of  a  close-set  network  of 
blood  capillaries.  These  capillaries  unite  in  a  small  blood- 
vessel which  runs  down  the  centre  of  each  lobule  towards  its 
base ;  this  central  blood-vessel  is  called  the  intralobular 
vein  (Fig.  72,  A,  h),  and,  passing  out  of  the  lobule  at  its 
base,  runs  into  a  branch  of  the  hepatic  vein  (Figs.  72  A,  s, 
and  73,  H.V.). 


Fig.  73.  —  A  Piece  of  the  Liver  cut  so  as  to  show 

//.  V.  a  branch  of  the  hepatic  vein,  L,  the  lobules  of  the  liver,  seated  upon  its  walls, 
and  sending  their  intralobular  veins  into  it. 


If  the  branches  of  the  hepatic  artery,  the  portal  vein, 
and  the  bile-duct  be  traced  into  the  substance  of  the  liver, 
they  will  be  found  to  accompany  one  another,  and  to 
branch  out  and  subdivide,  becoming  smaller  and  smaller. 
At  length  the  ultimate  branches  of  the  portal  vein  (Fig.  72, 


238  ELEMENTARY   PHYSIOLOGY  less. 

A,  p)  reach  the  outer  surfaces  of  the  lobules,  and  passing 
round  and  between  them  are  known  as  the  interlobular 
veins.  These  veins  pour  their  blood  into  the  network  of 
capillaries  which  permeates  each  lobule.  The  branches 
of  the  hepatic  artery  follow  a  course  parallel  to  that  of  the 
portal  vein  and  finally,  reaching  the  surface  of  a  lobule,  also 
pour  the  blood  they  carry  into  the  lobular  capillaries. 

Thus,  the  venous  blood  of  the  portal  vein  and  the  arterial 
blood  of  the  hepatic  artery  reach  the  surfaces  of  the  lobules 
by  the  ultimate  branches  of  that  vein  and  artery,  become 
mixed  in  the  capillaries  of  each  lobule,  and  are  carried  off 
by  its  intralobular  veinlet,  which  pours  its  contents  into  one 
of  the  branches  of  the  hepatic  vein.  These  branches,  join- 
ing together,  form  larger  and  larger  trunks,  which  at  length 
reach  the  hinder  margin  of  the  liver,  and  finally  open  into  the 
vena  cava  inferior,  where  it  passes  upwards  in  contact  with 
that  part  of  the  organ. 

Thus  the  blood  with  which  the  liver  is  supplied  is  a 
mixture  of  arterial  and  venous  blood  :  the  former  brought 
by  the  hepatic  artery  directly  from  the  aorta,  the  latter  by 
the  portal  vein  from  the  capillaries  of  the  stomach,  intes- 
tines, pancreas,  and  spleen. 

In  the  lobules  themselves  all  the  meshes  of  the  blood- 
vessels are  occupied,  as  has  been  said,  by  the  hepatic  cells 
or  liver  cells.  These  are  many-sided,  minute  bodies,  each 
about  20/x  (y^u-  of  an  inch)  in  diameter,  possessing  a 
nucleus  in  its  interior,  and  frequently  having  larger  and 
smaller  granules  of  fatty  matter  distributed  through  its 
substance  (Fig.  72,  A,  and  B,  a).  It  is  in  the  liver  cells 
that  the  active  powers  of  the  liver  reside. 

The  smaller  branches  of  the  hepatic  duct,  lined  by  an 
epithelium  which  is  continuous  with  that  of  the  main 
duct,  and   thence  with   that  of  the  intestines,  into  which 


vi  THE   WORK   OF  THE   LIVER  239 

ihc  main  duct  opens,  may  be  traced  to  the  very  surface  of 
the  lobules,  where  they  seem  to  end  abruptly  (Fig.  74). 
But,  upon  closer  examination,  it  is  found  that  they  com- 
municate with  a  network  of  minute  passages  passing  between 
the  hepatic  cells,  and  traversing  the  lobule  in  the  intervals 
left  by  the  capillaries  (Fig.  72,  B,  c).  These  minute 
passages  are  the  bile  canaliculi.  The  bile  manufactured  by 
the  hepatic  cells  finds  its  way  first  into  these  minute  pas- 
sages, from  them  into  the  ducts,  and  finally  either  into  the 
gall-bladder  or  the  intestines. 

17.  The  Work  of  the  Liver.  —  The  work  of  the  liver, 
and  this,  as  has  been  said,  is  carried  out  by  the  hepatic 
cells,  may  be  considered  as  consisting  of  two  kinds. 


Fig.   74.  —  Termination  of   Bile  Duct  at  Edge  of  Lobule. 
(Somewhat  diagrammatic.) 
b,  small  bile  duct,  becoming  still  smaller  at  b' ,  the  low,  flat  epithelium  at  last  sud- 
denly changing  into  the  hepatic  cells,  /,  the  channel  of  the  bile  duct  being  continued 
as  small  passages  between  the  latter;  c,  capillary  blood-vessels  cut  across. 

On  the  one  hand,  the  hepatic  cells  are  continually  en- 
gaged in  the  manufacture  of  a  complex  fluid  called  bile, 
which  they  pour  into  the  minute  passages  spoken  of  above, 
and  theme  into  the  branches  of  the  hepatic  duct,  whence 


240  ELEMENTARY   PHYSIOLOGY  less. 

it  flows  through  the  duct  itself  into  the  intestines,  or,  when 
digestion  is  not  going  on  and  the  opening  of  the  duct  into 
the  intestine  is  closed,  back  to  the  gall-bladder.  The 
materials  for  this  bile  are  supplied  to  the  hepatic  cells  by 
the  blood  ;  hence  the  secretion  of  the  bile  constitutes  a  loss 
to  the  blood. 

The  total  quantity  of  bile  secreted  in  the  twenty-four 
hours  varies,  but  probably  amounts  to  about  700  cubic 
centimetres  (1  pint).  It  is  a  golden  yellow,  slightly  alka- 
line fluid,  of  extremely  bitter  taste,  consisting  of  water  with 
from  15  per  cent,  to  half  that  quantity  of  solid  matter  in 
solution.  The  solids  consist  of  the  so-called  bile-pigments 
and  bile-salts,  a  remarkable  crystalline  substance  called 
cholesterin ;  a  small  quantity  of  fat ;  and  some  inorganic 
salts. 

The  colour  of  bile  is  due  to  the  pigment  called  bili- 
rubin. By  oxidation  this  may  easily  be  converted  into 
a  green  pigment  called  biliverdin,  and  the  differences  in 
colour  of  the  bile  of  different  animals  depend  on  the  rela- 
tive amounts  of  these  two  pigments  which  they  contain. 
The  bile-salts  are  sodium  salts  of  two  organic  acids,  one 
called  glycocholic,  the  other  taurocholic  acid.  The  former 
consists  of  carbon,  oxygen,  hydrogen,  and  nitrogen,  while 
the  latter  contains  additionally  a  considerable  quantity  of 
sulphur. 

Bile,  as  it  is  secreted  by  the  liver,  is  a  thin  fluid,  but  after 
its  sojourn  in  the  gall-bladder,  where  it  is  stored  in  the 
intervals  between  its  discharge  into  the  intestines,  it  contains 
a  considerable  amount  of  mucin,  secreted  into  it  by. the 
cells  which  line  the  gall-bladder,  and  it  is  then  viscid  and 
slimy. 

Of  these  constituents  of  the  bile  the  essential  substances, 
the  bile  acids  and  the  colouring  matter,  are  not  discoverable 


vi  THE   WORK   OF  THE   LIVER  241 

in  blood  which  enters  the  liver ;  they  must  therefore  be 
formed  in  the  hepatic  cells.  How  they  are  exactly  formed 
we  do  not  at  present  clearly  know.  The  material  of  which 
they  are  composed  is  brought  to  the  hepatic  cells  by  the 
blood,  but  the  exact  condition  of  that  material  —  whether, 
for  instance,  the  blood  brings  something  very  like  the  bile 
acids,  and  only  needing  a  slight  change  to  be  converted  into 
bile  acids  ;  or  whether  the  hepatic  cells  manufacture  the 
bile  acids  from  the  beginning,  as  it  were,  out  of  the  com- 
mon material  which  the  blood  brings  to  the  liver  as  to  all 
other  tissues  and  organs  —  is  not  as  yet  quite  determined. 
There  is,  however,  but  little  doubt  that  the  pigment  of  bile  is 
in  some  way  made  out  of  the  haemoglobin  of  the  red  blood- 
corpuscles  (see  p.  131).  The  saline  matters  and  choles- 
terin,  on  the  other  hand,  appear  to  be  present  in  the  blood 
of  the  portal  vein,  and  may  therefore,  like  the  water,  be 
simply  taken  up  by  the  cells  from  the  blood,  and  passed  on 
to  the  bile  ducts. 

Thus  the  bile  is  a  continual  loss  to  the  blood.  But, 
besides  forming  bile,  the  hepatic  cells  are  concerned  in 
other  labours,  the  result  of  which  can  hardly  be  considered 
either  as  a  loss  or  as  a  gain,  since  these  labours  simply  con- 
sist in  manufacturing  from  the  blood  and  storing  up  in  the 
hepatic  cells  substances  which,  sooner  or  later,  are  returned, 
generally  in  a  changed  condition,  back  into  the  blood. 

As  we  shall  presently  see,  the  portal  blood  is,  after  a  meal, 
heavily  laden  with  substances,  the  result  of  the  digestive 
changes  in  the  alimentary  canal.  When  these  substances, 
carried  along  in  the  portal  blood,  reach  the  hepatic  cells, 
in  the  meshes  of  the  lobules,  some  of  them  appear  to  be 
taken  up  by  those  cells  and  to  be  stored  up  in  them  in  a 
changed  condition.  In  fact,  the  products  of  digestion  pass-- 
ing  along  the  portal  veins  suffer  (in  the  liver)    a  further 

R 


242  ELEMENTARY   PHYSIOLOGY  less. 

change,  which  has  been  called  a  secondary  digestion. 
Thus  the  liver  produces  a  powerful  effect  on  the  quality 
of  the  blood  passing  through  it,  so  that  the  blood  in  the 
hepatic  vein  is  very  different,  especially  after  a  meal,  from 
the  blood  in  the  portal  vein. 

The  changes  thus  effected  by  the  hepatic  cells  are  prob- 
ably very  numerous,  but  they  have  not  been  fully  worked 
out,  except  in  one  particular  case,  which  is  very  interesting 
and  deserves  special  attention. 

It  is  found  that  the  liver  of  an  animal  which  has  been 
well  and  regularly  fed,  when  examined  immediately  after 
death,  contains  a  considerable  quantity  of  a  substance  which 
is  very  closely  allied  to  starch,  consisting  of  carbon,  hydro- 
gen, and  oxygen  in  proportions  the  same  as  in  starch.  This 
substance,  which  may  by  proper  methods  be  extracted  and 
preserved  as  a  white  powder,  is  in  fact  an  animal  starch, 
and  is  called  glycogen.  As  we  shall  see,  common  starch 
is  readily  changed  by  certain  agents  into  a  grape-sugar,  or 
dextrose,  as  it  should  be  called ;  and  this  glycogen  is  simi- 
larly converted  with  ease  into  dextrose.  Indeed,  if  the 
liver  of  such  an  animal  as  the  above,  instead  of  being  ex- 
amined immediately  after  death,  be  left  in  the  body,  or  be 
placed  on  one  side  after  removal  from  the  body  for  some 
hours  before  it  is  examined,  a  great  deal  of  the  glycogen 
will  have  disappeared,  a  quantity  of  dextrose  having  taken 
its  place.  There  seems  to  be  present  in  the  liver  some 
agent  capable  of  converting  the  glycogen  into  dextrose,  and 
this  change  is  particularly  apt  to  take  place  if  the  liver  is 
kept  at  blood-heat  or  near  that  temperature. 

Now  if,  instead  of  the  liver  of  a  well-fed  animal,  the  liver 
of  an  animal  which  has  fasted  for  several  days  be  examined 
in  the  same  way,  very  little  glycogen  indeed  will  be  found 
in  it,  and  when  this  liver  is  left  exposed  to  warmth  for  some 


vi  THE  WORK   OF  THE   LIVER  243 

time  very  little  dextrose  is  found.  That  is  to  say,  the  liver 
has,  in  the  first  case,  formed  the  glycogen  and  stored  it  up 
in  itself,  out  of  the  food  brought  to  it  by  the  portal  blood  : 
in  the  second  case,  no  food  has  been  brought  to  the  liver 
from  the  alimentary  canal,  no  glycogen  has  been  formed, 
and  none  stored  up.  If  the  liver  in  the  first  case  be  ex- 
amined microscopically  with  certain  precautions,  the  glyco- 
gen may  be  seen  stored  up  in  the  hepatic  cells  ;  in  the 
second  case  little  or  none  can  be  seen. 

The  kind  of  food  which  best  promotes  the  storing  up 
of  glycogen  in  the  liver  is  one  containing  starch  or  sugar ; 
but  some  glycogen  will  make  its  appearance  even  when  an 
animal  is  fed  on  an  exclusively  proteid  diet,  though  not 
nearly  so  much  as  when  starch  or  sugar  is  given. 

It  would  appear,  then,  that  the  hepatic  cells  can  manu- 
facture and  store  up  in  themselves  the  substance  glycogen, 
being  able  to  make  it  out  of  even  proteid  matter,  but  more 
easily  making  it  out  of  sugar ;  for,  as  we  shall  see,  all  the 
starch  which  is  eaten  as  food  is  converted  into  sugar  in  the 
alimentary  canal,  and  reaches  the  liver  as  sugar. 

There  are  reasons  for  thinking  that  the  glycogen,  thus 
deposited  and  stored  up  in  the  liver,  is  converted  into  sugar 
little  by  little  as  it  is  wanted,  poured  into  the  hepatic  vein, 
and  thus  distributed  over  the  body.  So  that  we  may  re- 
gard this  remarkable  formation  of  glycogen  in  the  liver  as 
an  act  by  which  the  blood,  when  it  is  over-rich  in  sugar, 
as  after  a  meal,  stores  it  up  or  deposits  it  in  the  liver  as 
glycogen ;  and  then,  in  the  intervals  between  meals,  the 
liver  deals  out  the  stored-up  material  as  sugar  back  again 
in  driblets  to  the  blood.  The  loss  to  the  blood,  therefore, 
is  temporary  —  no  more  a  real  loss  than  when  a  man  de- 
posits at  his  banker's  some  money  which  he  has  received, 
until  he  has  need  to  spend  it. 


^44  ELEMENTARY   PHYSIOLOGY  less. 

This  story  of  glycogen,  important  in  itself,  is  also  useful 
as  indicating  other  possible  effects  of  a  similar  nature  which 
the  hepatic  cells  may  bring  about  in  the  blood,  as  it  is 
passing  in  the  meshes  of  the  lobules  of  the  liver  from  the 
veinlets  of  the  portal  to  the  veinlets  of  the  hepatic  vein. 
The  formation  of  urea  by  the  hepatic  cells  has  already  been 
discussed  (p.  214).  Glycogen  and  urea  may  rightly  be 
spoken  of  as  internal  secretions  of  the  liver  (see  p.  201). 

18.  The  Spleen.  — The  spleen,  one  of  the  so-called  duct- 
less glands,  lies  in  the  abdominal  cavity,  slightly  below  and 
towards  the  left  side  of  the  stomach  and  immediately  to  the 
left  of  the  tail  of  the  pancreas  (Fig.  75,  SpL).  It  is  an 
elongated,  flattened,  red  body,  abundantly  supplied  with 
blood  by  an  artery  called  the  splenic  artery,  which  proceeds 
almost  directly  from  the  aorta.  The  blood  which  has  trav- 
ersed the  spleen  is  collected  by  the  splenic  vein,  and  is 
carried  by  it  to  the  portal  vein,  and  so  to  the  liver.  The 
spleen  is  covered  by  a  capsular  sheath  of  connective  tissue 
mixed  with  a  good  deal  of  elastic  tissue  and  some  unstriated 
muscle  fibres.  Somewhat  in  the  same  way  as  in  a  lymphatic 
gland  (p.  115)  this  capsule  sends  branching  projections  or 
trabeculae  inwards,  which  divide  the  organ  up  into  a  number 
of  irregular  spaces,  and  these  spaces  are  filled  with  a  mass 
of  spongy  tissue  called  the  spleen-pulp.  The  pulp  is 
traversed  by  a  network,  the  meshes  of  which  are  occupied 
by  red  blood-corpuscles,  by  colourless  corpuscles  closely 
similar  to  those  of  lymph,  and  by  other  kinds  of  cells  pecul- 
iar to  the  spleen.  The  latter,  or  spleen  cells,  resemble  the 
colourless  corpuscles  of  blood,  in  that  they  can  perforin 
amoeboid  movements,  but  they  are  larger  and  contain  in 
their  substance  red  corpuscles  in  various  stages  of  disinte- 
gration. 

A  section  of  the  spleen  shows  a  dark  red  spongy  mass 


THE   SPLEEN 


245 


dotted  over  with  minute  whitish  spots.  Each  of  these  last 
is  the  section  of  one  of  the  spheroidal  bodies  called  cor- 
puscles of  the  spleen,  or  Malpighian  corpuscles,  which  are 
scattered  through  its  substance.  These  corpuscles  consist 
of  little  masses  of  lymphoid  or  adenoid  tissue,  very  similar 
to  that  found  in  the  lymphatic  glands  (p.  116)  which  sur- 
round the  smaller  branches  of  the  arteries.  They  are 
crowded  with  leucocytes,  and  hence  they  stand  out  as 
white  specks  against  the  dark  red  pulp  of  the  spleen. 


Vrrv. 


Fig. 


75- 


The  spleen  (Spl.)  with  the  splenic  artery  {Sp.  A.).  Below  this  is  seen  the 
splenic  vein  running  to  help  to  form  the  portal  vein  (  V.  P.).  Ao,  the  aorta;  D.  a 
pillar  of  the  diaphragm;  P.D.  the  pancreatic  duct  exposed  by  dissection  in  the  sub- 
stance of  the  pancreas;  Dm.  the  duodenum;  B.D.  the  biliary  duct  uniting  with  the 
pancreatic  duct  into  the  common  duct,  x;  y,  the  intestinal  vessels. 


The  smallest  branches  of  the  arteries  which  carry  blood 
into  the  spleen  open  into  the  network  of  the  spleen-pulp, 
so  that  the  blood  flows  into  and  through  this  network ;  it  is 
then  gathered  up  again  into  the  ends  of  tiny  veins,  which 
similarly  open  into  the  spleen-pulp,  and  carry  the  blood 
away  into  the  splenic  vein. 


246  ELEMENTARY   PHYSIOLOGY  less. 

We  are  still  very  much  in  the  dark  as  to  the  functions  of 
the  spleen;  they  are  without  doubt  of  some  importance  ;  but, 
on  the  other  hand,  the  spleen  may  be  permanently  removed 
from  the  body  without  producing  any  obvious  derangement 
of  its  working. 

The  elasticity  of  the  splenic  tissue  allows  the  organ  to  be 
readily  distended  with  blood,  and  enables  it  to  return  to  its 
former  size  after  distension.  It  appears  to  change  its  dimen- 
sions with  the  state  of  the  abdominal  viscera,  attaining  its 
largest  size  about  five  hours  after  a  full  meal,  and  gradually 
returning  to  its  minimum  bulk. 

The  blood  of  the  splenic  vein  is  found  to  contain  more 
colourless  corpuscles  than  that  of  the  splenic  artery ;  and  it 
has  been  supposed  that  the  spleen  is  one  of  those  parts  of 
the  economy  in  which  colourless  corpuscles  of  the  blood  are 
produced.  It  is  also  thought  that  red  corpuscles  there  die 
and  are  broken  up. 

19.  The  Thymus  Gland.  —  This  ductless  gland  lies  over 
the  trachea,  in  the  lower  part  of  the  neck  and  behind  the 
sternum  at  the  base  of  the  heart.  It  is  conspicuous  at  birth, 
but  soon  begins  to  waste  away,  and  in  the  adult  is  replaced 
by  a  small  amount  of  connective  tissue  and  fat.  In  structure 
it  somewhat  resembles  a  lymphatic  gland. 

Nothing  definite  is  known  of  the  function  or  use  of  this 
gland. 

20.  The  Thyroid  Body  or  Gland.  —  This  organ  consists 
of  two  lobes,  one  lying  on  each  side  of  the  trachea,  just  be- 
low the  larynx  and  the  two  being  joined  across  the  trachea 
by  a  connecting  strip  of  thyroid  tissue.  Each  lobe  is  cov- 
ered with  a  capsule  of  connective  tissue,  from  which  branches 
pass  inwards  and  divide  the  interior  into  rounded  spaces  or 
alveoli.  Each  alveolus  is  lined  by  a  layer  of  cubical  cells 
so  as  to  leave  a  large  central  closed  space,  which  is  filled 


vi  THE   SUPRARENAL   BODIES  247 

with  a  clear,  viscid,  often  semi-solid  fluid.  The  body  pos- 
sesses no  duct. 

The  thyroid  gland  seems  to  have  much  to  do  with  the 
nutrition  of  the  body.  When  diseased  in  man,  it  often  leads 
to  nutritive  disorders,  strikingly  manifest  in  a  puffed,  swollen 
appearance  of  the  skin,  but  involving  various  organs  and 
tissues,  especially  the  nervous  system,  and  thus  leading  to 
nervous  troubles.  Occasionally  the  degenerations  of  the 
tissues  take  on  the  form  of  a  change  into  a  mucin-like 
substance.  These  troubles  may  be  largely  mitigated  by 
taking  doses  of  an  extract  of  the  fresh  gland  or  by  eating 
the  fresh  gland-substance.  Goitre  is  an  enlargement  of  the 
thyroid,  and  cretinism,  a  peculiar  form  of  idiocy  common 
in  some  places,  is  associated  with  its  diseased  condition. 
Recent  experimental  work  seems  to  show  beyond  a  doubt 
that  the  thyroid  secretes  material  which  passes  into  the  cir- 
culation and  is  of  use  to  the  organism.  The  nature  of  this 
internal  secretion  is  not  definitely  known.  Especially  sig- 
nificant, however,  is  the  presence  in  the  gland  of  iodine, 
apparently  in  an  organic  compound,  which  has  been  called 
iodo-thyrin.  This  compound  is  thought  to  be  the  active 
constituent  of  the  internal  secretion.  Disease  of  the  gland 
doubtless  interferes  with  its  production  and  thus  causes  the 
characteristic  disorders. 

21.  The  Suprarenal  Bodies.  —  The  suprarenal  bodies  are 
two  in  number,  and  are  placed  one  on  the  upper  edge  of  each 
kidney.  They  are  enveloped  in  an  outer  coat,  or  capsule. 
of  connective  tissue,  from  which  partitions  pass  into  theii 
interior,  dividing  it  up  into  compartments.  The  spaces  in 
the  cortical  part  are  filled  with  groups  of  angular  cells ; 
those  of  the  medullary  part  by  cells  larger  and  more  irregu- 
lar in  shape. 

The  functions   of  the  suprarenal  bodies   are   important 


248  ELEMENTARY   PHYSIOLOGY  less,  vi 

although  as  yet  but  little  understood.  When  they  are 
both  removed  from  an  animal,  death  speedily  ensues,  ac- 
companied chiefly  by  great  muscular  weakness.  When  dis- 
eased in  man,  similar  weakness  is  observed,  together  with  a 
characteristic  "bronzing"  or  coloration  of  the  skin.  Extract 
of  the  suprarenals,  when  injected  into  the  body,  has  a  power- 
fully stimulating  effect  upon  the  muscular  system,  especially 
the  muscles  of  the  heart  and  the  arteries,  and  thus  causes  a 
great  increase  of  arterial  pressure.  Such  extract  probably 
will  be  found,  as  in  the  case  of  the  thyroid  gland,  to  mitigate 
the  symptoms  which  result  from  the  bodies  being  diseased. 
As  in  the  case  of  the  thyroid  also,  the  facts  known  regarding 
the  suprarenals  are  interpreted  as  indicating  that  these  bodies 
constantly  secrete  into  the  blood  in  minute  quantities  a  sub- 
stance or  substances  that  are  beneficial  to  the  body,  espe- 
cially to  the  muscular  system.  In  fact,  an  organic  compound 
has  been  obtained  from  the  bodies  which,  when  injected 
into  a  living  animal,  has  an  effect  upon  the  blood-vessels 
similar  to  that  of  extracts  of  the  bodies  themselves.  To 
this  compound  the  name  epinephrin  has  been  given.  But 
much  remains  to  be  discovered  regarding  not  only  the  func- 
tions of  these  and  other  ductless  glands,  but  the  general 
physiology  of  internal  secretion  itself. 


LESSON   VII 

THE  SOURCES  OF  LOSS  AND  GAIN  TO  THE  BLOOD 
(continued)  :  THE  FUNCTION  OF  ALIMENTATION 

Part  I.  —  Digestion  and  Absorption 

1.  "Waste  made  Good  by  Food.  —  We  explained  in  the 
first  Lesson  that  a  living  active  man  is  always  expending 
energy  in  the  form  of  the  mechanical  (muscular)  work  he 
performs  and  of  the  heat  he  gives  off  by  his  skin  and  lungs. 
Further,  we  pointed  out  that  the  source  from  which  the 
energy  is  derived  lies  in  that  constant  oxidational  breaking 
down  of  the  tissues  which  results  from  their  being  supplied 
with  oxygen,  introduced  into  the  body  by  the  lungs.  And, 
further,  it  was  shown  that  the  above  processes  result  in  a 
waste  of  substance  corresponding  exactly  to  the  amount  of 
energy  expended.  If  the  man's  activity  is  to  continue  from 
day  to  day,  this  continual  waste  of  substance  must  be  made 
good.  Now  the  only  channel,  except  the  lungs,  by  which 
altogether  new  material  is  introduced  into  the  body,  is  the 
alimentary  canal,  and  we  may  use  the  word  alimentation  to 
denote  the  sum  total  of  its  operations  in  this  connection. 
These  fall  naturally  under  three  heads,  viz.  the  introduction 
of  food  as  new  material ;  the  reduction  of  this  food  by 
digestion  to  a  condition  such  that  it  can  pass  through  the 
delicate  structures  which  form  the  walls  of  the  vessels 
of  the  alimentary  canal;   and  absorption,  or  the  processes 

249 


250  ELEMENTARY    THYSIOLOGY  less. 

by  which  the  digested  material  is  passed  from  the  cavity 
of  the  canal  into  the  blood-vessels  and  lymphatics,  by 
which  it  is  then  distributed  over  the  body.  We  may  there- 
fore most  suitably  begin  by  learning  something  of  the 
nature  and  composition  of  that  "new  material"  which  we 
introduce  into  the  body  as  food. 

2.  Food  and  Food- stuffs.  —  Every  one  is  familiar  with 
the  meaning  of  the  term  food,  as  exemplified  by  bread, 
meat,  potatoes,  milk,  etc.  None  of  these  substances, 
however,  is  made  up  of  one  kind  of  material ;  but  when 
analysed  it  is  found  that  they  all  consist  of  varying  amounts 
of  a  few  substances,  and  to  these  the  name  of  food-stuffs  is 
given. 

Food-stuffs  are  classified  under  four  heads,  (1)  Proteids, 
(2)  Fats,  (3)  Carbohydrates,  (4)  Salts  (mineral  matter) 
and  Water.  They  may  further  be  divided  into  two  distinct 
groups  :  —  the  nitrogenous  and  the  non-nitrogenous.  The 
proteids  alone  contain  nitrogen  and  thus  form  one  group  by 
themselves ;  the  other  food-stuffs  are  all  non-nitrogenous. 
Further,  the  first  three  classes,  as  being  compounds  of 
carbon,  are  known  as  organic  compounds,  while  the  salts 
and  water  are  inorganic.  They  may  therefore  be  tabulated 
as  follows  :  — 

Organic  Inorganic 


(Nitrogenous)     (Non-nitrogenous)  (Non-nitrogenous) 

II  I 

Proteids                       Fats  Salts 

I  I 

Carbohydrates  Water 

A.   Nitrogenous  Food-stuffs. 

Proteids.  —  These   are    composed  of  the    four  elements 
carbon,  oxygen,  hydrogen,  and  nitrogen,  united  with  small 


vii  NON-NITROGENOUS   FOOD-STUFFS  25: 

amounts  of  sulphur  and  frequently  of  phosphorus  (see 
p.  134).  Under  this  head  come  the  albumin  of  the  white 
of  egg  and  of  blood-serum  ;  the  casein  of  milk  and  cheese  ; 
the  gluten  of  flour  and  other  cereals ;  the  myosin  of  lean 
meat  (muscle)  ;  the  globulins  of  blood  and  of  the  yolk  of 
an  egg  ;  and  the  fibrin  of  blood. 

Gelatin,  the  basis  of  connective  tissue  fibres,  is  composed 
of  the  same  elements  as  a  proteid  and  in  somewhat  similar 
proportions,  and  may  be  regarded  as  an  outlying  member 
of  this  group.  But  gelatin  is  not  a  true  proteid  and  cannot 
entirely  replace  it  in  food. 

B.   Non-nitrogenous  Food-stuffs. 

(i)  Fats.  —  These  are  composed  of  carbon,  oxygen,  and 
hydrogen  only,  and  contain  less  oxygen  than  would  form 
water  if  united  to  the  hydrogen  they  contain.  Butter 
and  all  animal  and  vegetable  oils  come  under   this  head. 

(ii)  Carbohydrates.  —  These  are  substances  which  also 
consist  of  carbon,  oxygen,  and  hydrogen  only,  but  in  them 
the  oxygen  is  present  in  an  amount  which  would  just  suffice 
to  form  water  if  it  were  united  to  their  hydrogen.  This 
group  includes  starch,  as  in  flour  and  potatoes ;  ordinary 
cane-sugar  or  beet-sugar,  and  other  sugars  such  as  dextrose 
and  milk-sugar  ;  also  cellulose  from  all  vegetable  tissues. 

(iii)  Salts  and  Water.  —  Water  is  present  in  all  foods,  and 
salts  in  most  of  them,  such  as  meat,  eggs,  milk,  and  cheese. 
The  salts  are  chiefly  the  phosphates,  chlorides,  and  carbon- 
ates of  sodium,  potassium,  and  calcium,  and  some  salts 
of  iron. 

All  food  is  made  up  of  these  food-stuffs,  but  the  amount 
of  each  present  in  different  foods  varies  greatly.  Thus 
lean  meat  is  chiefly  proteid,  but  ordinarily  contains  a  good 
deal  of  fat ;    bread  contains  a  great  deal  of  carbohvdrates, 


252  ELEMENTARY   PHYSIOLOGY  less. 

but  also  some  proteid  and  a  little  fat.  Only  the  fats  and 
oils  may  be  regarded  as  composed  of  nearly  pure  material. 
The  composition  of  the  chief  foods  is  important  and  has 
been  carefully  determined ;  and  to  this  we  shall  return 
when  we  come  to  study  their  respective  influences  on  the 
body  as  a  whole. 

3.  The  Purpose  and  Means  of  Digestion.  —  All  food- 
stuffs being  thus  proteids,  fats,  carbohydrates,  or  mineral 
matters,  pure  or  mixed  up  with  other  substances,  the  whole 
purpose  of  the  alimentary  apparatus  is  in  the  first  place  to 
separate  these  proteids,  etc.,  from  the  innutritious  residue, 
if  there  be  any,  and  to  reduce  them  into  a  condition  either 
of  solution  or  of  excessively  fine  subdivision,  in  order  that 
they  may  make  their  way  through  the  delicate  structures 
which  form  the  walls  of  the  vessels  of  the  alimentary  canal. 
In  the  next  place  this  mechanical  and  physical  change  must 
be  accompanied  by  chemical  changes  whereby  the  food- 
stuffs are  brought  into  such  a  condition  that  when  they 
reach  the  tissues  the  latter  can  take  them  up  or  assimilate 
them. 

To  these  ends  food  is  taken  into  the  mouth  and  masti- 
cated, is  mixed  with  saliva,  is  swallowed,  undergoes  gas- 
tric digestion,  passes  into  the  intestine,  and  is  subjected  to 
the  action  of  the  secretions  of  the  liver  and  pancreas  with 
which  it  there  becomes  mixed ;  and,  finally,  after  the  more 
or  less  complete  extraction  of  the  nutritive  constituents,  the 
residue,  mixed  up  with  certain  secretions  of  the  intestines, 
leaves  the  body  as  the  faeces. 

The  actual  digestive  changes  of  food  are  brought  about 
chiefly  by  the  action  of  fluids  secreted  by  glands  whose 
ducts  pour  their  secretions  into  the  cavity  of  the  alimentary 
canal. 

4.  The  Mouth  and  the  Teeth.  —  The  cavity  of  the  mouth 


vii  THE   MOUTH   AND   THE  TEETH  253 

is  a  chamber  with  a  fixed  roof,  formed  by  the  hard  palate 
(Fig.  76,  /),  and  with  a  movable  floor,  constituted  by  the 
lower  jaw,  and  the  tongue'  (k),  which  fills  up  the  space  be- 
tween the  two  branches  of  the  jaw.  Arching  round  the 
margins  of  the  upper  and   the  lower  jaws  are  thirty-two 


Fig.  76.  —  A  Section  of  the  Mouth  and  Nose  taken  vertically,  a  little 
to  the  left  of  the  Middle  Line. 

a,  the  vertebral  column;  6,  the  oesophagus  or  gullet;  c,  the  trachea  or  windpipe, 
d,  the  thyroid  cartilage  of  the  larynx;  e,  the  epiglottis:  /*,  the  uvula;  g,  the  opening 
of  the  left  Eustachian  tube;  h,  the  opening  of  the  left  lachrymal  duct;  i,  the  hyoid 
bone;  k,  the  tongue;  /,  the  hard  palate;  m,  «,  the  base  of  the  skull;  o,  p,  g,  the 
superior,  middle  and  inferior  turbinal  bones.  The  letters  g,f,  e,  are  placed  in  the 
pharynx. 

teeth,  sixteen  above  and  sixteen  below,  and  external  to 
these,  the  closure  of  the  cavity  of  the  mouth  is  completed 
by  the  cheeks  at  the  sides,  and  by  the  lips  in  front. 


254  ELEMENTARY    PHYSIOLOGY  less. 

AYhen  the  mouth  is  shut  the  back  of  the  tongue  comes 
into  close  contact  with  the  palate  ;  and,  where  the  hard 
palate  ends,  the  communication  between  the  mouth  and 
the  back  of  the  throat  is  still  further  impeded  by  a  sort  of 
fleshy  curtain  —  the  soft  palate  or  velum  —  the  middle  of 
which  is  produced  into  a  prolongation,  the  uvula  (/),  while 
its  sides,  skirting  the  sides  of  the  passage,  or  fauces,  form 
double  muscular  pillars,  which  are  termed  the  pillars  of  the 
fauces.  Between  these  the  tonsils  are  situated,  one  on  each 
side. 

The  velum  with  its  uvula  comes  into  contact  below  with 
the  upper  part  of  the  back  of  the  tongue,  and  with  a  sort 
of  gristly,  lid-like  process  connected  with  its  base,  the 
epiglottis  (e). 

Behind  the  partition  thus  formed  lies  the  cavity  of  the 
pharynx,  which  may  be  described  as  a  funnel-shaped  bag 
with  muscular  walls,  the  upper  margins  of  the  slanting,  wide 
end  of  which  are  attached  to  the  base  of  the  skull,  while  the 
lateral  margins  are  continuous  with  the  sides,  and  the  lower 
with  the  floor,  of  the  mouth.  The  narrow  end  of  the  pharyn- 
geal bag  passes  into  the  gullet  or  oesophagus  (<£),a  muscular 
tube  which  affords  a  passage  into  the  stomach. 

There  are  no  fewer  than  six  distinct  openings  into  the 
front  part  of  the  pharynx  —  four  in  pairs,  and  two  single 
ones  in  the  middle  line.  The  two  pairs  are,  in  front,  the 
hinder  openings  of  the  nasal  cavities ;  and  at  the  sides, 
close  to  these,  the  apertures  of  the  Eustachian  tubes  (g), 
which  connect  the  pharynx  with  the  middle  ears.  The  two 
single  apertures  are,  the  hinder  opening  of  the  mouth  be- 
tween the  soft  palate  and  the  epiglottis ;  and,  behind  the 
epiglottis,  the  upper  aperture  of  the  respiratory  passage, 
or  the  glottis. 

Each  tooth  presents  a  crown,  which  is  visible  in  the  cavity 


vii  THE   MOUTH   AND  THE  TEETH  255 

of  the  mouth,  where  it  becomes  worn  by  attrition  with  the 
tooth  opposite  to  it  and  with  the  food  ;  and  one  or  more 
fangs,  which  are  buried  in  a  socket  or  alveolus,  furnished  by 
the  jaw-bone  and  the  dermis  of  the  dense  mucous  membrane 
of  the  mouth.  This  covering  of  the  jaw-bone  constitutes  the 
gum.  The  line  of  junction  between  the  crown  and  the  fang 
is  the  neck  of  the  tooth. 

The  eight  teeth  on  opposite  sides  of  the  same  jaw  are  con- 
structed upon  exactly  similar  patterns,  while  the  eight  teeth 
which  are  opposite  to  one  another  and  bite  against  one  an- 
other above  and  below,  though  similar  in  kind,  differ  some- 
what in  the  details  of  their  patterns. 

The  two  teeth  in  each  eight  which  are  nearest  the  middle 
line  in  the  front  of  the  jaw,  have  wide,  but  sharp  and  chisel- 
like, edges.  Hence  they  are  called  incisors,  or  cutting  teeth. 
The  tooth  which  comes  next  is  a  tooth  with  a  more  conical 
and  pointed  crown.  It  answers  to  the  great  tearing  and 
holding  tooth  of  the  dog,  and  is  called  the  canine  or  eye- 
tooth.  The  next  two  teeth  have  broader  crowns,  with  two 
cusps,  or  points,  on  each  crown,  one  on  the  inside  and  one 
on  the  outside,  whence  they  are  termed  bicuspid  teeth,  and 
sometimes  false  grinders.  All  these  teeth  have  usually  one 
fang  each,  except  the  bicuspid,  the  fangs  of  which  may  be 
more  or  less  completely  divided  into  two.  The  remaining 
teeth  have  two  or  three  fangs  each,  and  their  crowns  are 
much  broader.  Since  they  crush  and  grind  the  matters  which 
pass  between  them  they  are  called  molars,  or  true  grinders. 

In  the  interior  of  the  tooth  is  a  cavity  communicating 
with  the  exterior  by  canals,  which  traverse  the  fangs  and 
open  at  their  points.  This  cavity  is  the  pulp  cavity  (Fig. 
77,  l>).  It  is  occupied  and  completely  filled  by  a  highly 
vascular  tissue  richly  supplied  with  nerves,  the  dental  pulp, 
which  is  continuous  below,  through  the  opening  of  the  fangs, 


250 


ELEMENTARY   PHYSIOLOGY 


with  the  vascular  dermis  of  the  gum  which  lies  between  the 
fangs  and  the  alveolar  walls,  and  plays  the  part  of  periosteum 
to  both. 

The  tissue  which  forms  the  chief  constituent  of  a  tooth  is 
termed  dentine  (Fig.  77,  d).  It  is  a  dense  and  calcified 
substance  containing  less  animal  matter  than  bone,  perme- 
ated by  innumerable,  minute,  parallel,  wavy  tubules  (Fig. 
78,  a),  which  give  off  lateral  branches.  The  wider  inner 
ends  of  these  tubules  measure  on  the  average  5^1  (45^0 
inch)   in  diameter  ■    they  open  into  the  pulp  cavity,  while 


Fig.   77. 

A,  vertical,  B,  horizontal  section  of  a  tooth,  a,  enamel  of  the  crown;  b,  pulp 
cavity;  c,  cement  of  the  fangs ;  d,  dentine.     (Magnified  about  three  diameters.) 

the  narrower  outer  terminations  ramify  at  the  surface  of  the 
dentine,  and  may  even  extend  into  the  enamel  or  cement 
(Fig.  78). 

The  greater  part  of  the  crown  and  almost  the  whole  of  the 
fangs  consist  of  dentine.  But  the  summit  of  the  crown  is 
invested  by  a  thick  layer  of  a  much  denser  tissue,  which  con- 
tains only  2  per  cent,  of  animal  matter,  and  is  the  hardest 


vii  THE   DEVELOPMENT   OF  THE  TEETH  257 

substance  in  the  body ;  so  hard  that  it  will  strike  fire  with 
steel.  This  is  called  enamel  (Fig.  77,  a).  It  becomes 
thinner  on  the  sides  of  the  crown  and  gradually  dies  out  on 
the  neck.  Examined  microscopically,  the  enamel  is  seen  to 
consist  of  six-sided  prismatic  fibres  (Fig.  78,  A,  B)  set 
closely  side  by  side,  nearly  at  right  angles  to  the  surface  of 
the  dentine.  These  fibres  measure  about  5 /a  (-g-gVg-  inch) 
in  transverse  diameter  and  present  transverse  striations. 

The  third  tissue  found  in  teeth  is  a  thin  layer  of  true  bone, 
generally  devoid  of  Haversian  canals,  which  invests  the 
outer  surface  of  the  fangs  and  thins  out  on  the  neck.  This 
is  termed  cement  (Fig.  77,  A,  c ;  and  Fig.  78,  C,  e). 

The  dental  pulp  is  chiefly  composed  of  delicate  connec- 
tive tissue.  It  is  abundantly  supplied  with  vessels  and  nerves, 
which  enter  it  through  the  small  opening  at  the  extremity 
of  the  fang.  The  nerves  are  mainly  sensory  branches  de- 
rived from  the  fifth  pair  of  cranial  nerves  (Lesson  XII). 

The  superficial  part  of  the  pulp,  which  is  everywhere  in 
immediate  contact  with  the  inner  surface  of  the  dentine, 
consists  of  a  layer  of  nucleated  cells  so  close  set  that  they 
almost  resemble  an  ■  epithelium.  They  are,  however,  in 
reality  connective  tissue  cells  (and  the  layer  is  merely  a 
slightly  modified  condition  of  the  stratum  of  undifferen- 
tiated connective  tissue  which  lies  at  the  surface  of  every 
dermal  structure),  and  from  them  long  filamentous  processes 
can  be  traced  into  the  dentinal  tubules. 

5.  The  Development  of  the  Teeth.  —  The  teeth  begin  to 
be  developed  long  before  birth,  and  while  the  jaw-bones  are 
in  a  very  rudimentary  condition.  The  epithelium  covering 
the  gums  thickens  into  a  ridge  and  grows  down  into  the  un- 
derlying dermis,  which  at  the  same  time  grows  up  at  the 
sides  of  the  ridge.  In  this  way  a  semicircular  groove,  the 
dental  groove,  is  developed  in  the  dermis  of  the  gum  of 


258  ELEMENTARY   PHYSIOLOGY  less. 

each  jaw.  The  epithelium  of  the  gum,  however,  completely 
fills  the  groove  and  passes  from  side  to  side  smoothly  over 
it.  Next,  each  groove  becomes  subdivided  into  ten  pouches, 
five  on  each  side  of  the  middle  line,  and  behind  the  fifth  on 
each  side  there  remains  a  residue  of  the  groove,  which  may 
be  called  a  residual  pouch. 

Each  of  the  first-mentioned  pouches  becomes  gradually 
more  and  more  distinct  from  its  neighbours,  until  at  length 
its  walls  unite  and  shut  off  the  epithelium  which  it  contains 
from  the  cavity  of  the  mouth.  The  result  is  a  closed  bag 
full  of  epithelium,  which  is  a  milk  tooth  sac.  At  the  same 
time  the  dermis  of  the  bottom  of  the  sac  has  grown  up  as  a 
conical  process  into  its  interior;  and  this  dental  papilla  is 
the  rudiment  of  the  future  tooth. 

While  the  milk-tooth  sac  is  thus  shaping  itself,  its  epithe- 
lium grows  out  on  one  side  into  a  small  process,  which  gradu- 
ally increases  in  size  and  takes  on  the  characters  of  a  second 
tooth  sac.  This  is  the  sac  of  the  permanent  tooth,  which 
answers  to  and  will  replace  each  milk  tooth. 

A  similar  change  takes  place  in  the  residual  pouches,  each 
of  which  gradually  becomes  divided  into  three  sacs  for  the 
three  hindmost  permanent  teeth  in  each  jaw. 

The  sacs  of  the  milk  teeth  rapidly  increase  in  size  and 
become  separated  from  one  another  by  partitions  of  bone 
developed  from  the  jaw  with  which  they  are  in  relation,  and 
which  grow  up  round  them.  They  thus  become  lodged  in 
alveoli. 

The  proper  tooth  substance  first  makes  its  appearance  as 
a  very  thin  hollow  cap  of  glassy  calcareous  deposit  at  the 
summit  of  the  papilla.  This  cap  gradually  extends  and  in- 
creases in  thickness,  the  increase  of  the  tooth  being  accom- 
panied by  decrease  of  the  papilla,  which  eventually  remains 
in  the  cavity  of  the  finished  tooth  as  the  pulp. 


THE    DEVELOPMENT   OF   THE   TEETH 


259 


The  fully  formed  milk  teeth  press  upon  the  upper  walls 
of  the  sacs  in  which  they  are  inclosed,  and,  causing  a  more 


Fig.   78. 

A.  Enamel  fibres  viewed  in  transverse  section. 

B.  Enamel  fibres  separated  and  viewed  laterally. 

C.  A  section  of  a  tooth  at  the  junction  of  the  dentine  (a)  with  the  cement  (e); 
i,  c,  irregular  cavities  in  which  the  tubules  of  the  dentine  end;  d,  fine  tubules  con- 
tinued from  them;  f,  g,  lacuna;  and  canaliculi  of  the  cement.  (Magnified  about  40c 
diameters.) 

or  less  complete  absorption  of  these  walls,  force  their  way 
through.     The  teeth  are  then,  as  it  is  called,  cut. 


26o  ELEMENTARY   PHYSIOLOGY  less. 

The  cutting  of  this  first  set  of  teeth,  called  deciduous,  or 
milk  teeth,  commences  at  about  six  months,  and  ends  with 
the  second  year.  They  are  altogether  twenty  in  number  — 
eight  being  cutting  teeth,  or  incisors ;  four,  eye  teeth,  or 
canines  ;  and  eight,  grinders,  or  molars. 

It  has  been  seen  that  each  dental  sac  of  the  milk  teeth,  as 
it  is  formed,  gives  off  a  little  prolongation  ;  this  becomes 
lodged  in  the  jaw  below  the  milk  tooth,  enlarges,  and  de- 
velops a  papilla  from  which  a  new  tooth  is  formed.  As  the 
latter  increases  in  size,  it  presses  upon  the  root  of  the  milk 
tooth  which  preceded  it,  and  thereby  causes  the  absorption 
of  the  root  and  the  final  falling  out,  or  shedding,  of  the  milk 
tooth,  whose  place  it  takes.  Thus  every  milk  tooth  is  re- 
placed by  a  tooth  of  what  is  termed  the  permanent  dentition. 
The  permanent  incisors  and  canines  are  larger  than  the  milk 
teeth  of  the  same  name,  but  otherwise  differ  little  from  them. 
The  permanent  teeth,  which  replace  the  milk  molars,  are  the 
bicuspids. 

We  have  thus  accounted  for  twenty  of  the  teeth  of  the 
adult.  The  permanent  back  grinders,  or  molars,  are  de- 
veloped in  the  sacs  which  are  formed  out  of  the  residual 
pouches  above  mentioned.  The  first  of  these  teeth,  the 
anterior  molar  of  each  side,  is  the  earliest  cut  of  all  the 
permanent  set,  and  appears  at  six  years  of  age.  The  last, 
or  hindermost,  molar  is  the  last  of  all  to  be  cut,  usually  not 
appearing  till  twenty-one  or  twenty-two  years  of  age.  Hence 
it  goes  by  the  name  of  the  "  wisdom  tooth." 

6.  Mastication. — The  muscles  of  the  parts  which  have 
been  described  have  such  a  disposition  that  the  lower  jaw 
can  be  depressed,  so  as  to  open  the  mouth  and  separate  the 
teeth  ;  or  be  raised,  in  such  a  manner  as  to  bring  the  teeth 
together  \  or  move  obliquely  from  side  to  side,  so  as  to  cause 
the  face  of  the  grinding  teeth  and  the  edges  of  the  cutting 


vii  THE  CESOPHAGUS   AND   SWALLOWING  261 

teeth  to  slide  over  one  another.  And  the  muscles  which 
perform  the  elevating  and  sliding  movements  are  of  great 
strength,  and  confer  a  corresponding  force  upon  the  grind- 
ing and  cutting  actions  of  the  teeth. 

When  solid  food  is  taken  into  the  mouth,  it  is  cut  and 
ground  by  the  teeth,  the  fragments  which  ooze  out  upon  the 
outer  side  of  their  crowns  being  pushed  beneath  them  again 
by  the  muscular  contraction  of  the  cheeks  and  the  lips  ;  while 
those  which  escape  on  the  inner  side  are  thrust  back  by  the 
tongue,  until  the  whole  is  thoroughly  rubbed  down. 

While  mastication  is  proceeding,  the  salivary  glands  pour 
out  their  secretion  in  great  abundance,  and  the  saliva  mixes 
with  the  food,  which  thus  becomes  interpenetrated  not  only 
with  the  salivary  fluid,  but  with  the  air  which  is  entangled  in 
the  bubbles  of  the  saliva. 

7.  The  (Esophagus  and  Swallowing.  —  When  the  food 
is  sufficiently  ground  it  is  collected,  enveloped  in  saliva,  into 
a  mass  or  bolus,  which  rests  upon  the  back  of  the  tongue, 
and  is  carried  backwards  to  the  aperture  which  leads  into 
the  pharynx.  Through  this  it  is  thrust,  the  soft  palate  being 
lifted  and  its  pillars  being  brought  together,  while  the  back- 
ward movement  of  the  tongue  at  once  propels  the  mass  and 
causes  the  epiglottis  to  incline  backwards  and  downwards 
over  the  glottis  and  so  to  form  a  bridge,  by  which  the  bolus 
can  travel  over  the  opening  of  the  air-passage  without  any 
risk  of  tumbling  into  it.  While  the  epiglottis  directs  the 
course  of  the  mass  of  food  below,  and  prevents  it  from  pass- 
ing into  the  trachea,  the  soft  palate  guides  it  above,  keeps 
it  out  of  the  nasal  chamber,  and  directs  it  downwards  and 
backwards  towards  the  lower  part  of  the  muscular  pharyngeal 
funnel.  By  this  the  bolus  is  immediately  seized  and  tightly 
held,  and  the  muscular  fibres  contracting  above  it,  while 
they  are  comparatively  lax  below,  it  is  rapidly  thrust  into 
and  down  the  oesophagus. 


262  ELEMENTARY   PHYSIOLOGY  less, 

The  oesophagus  is  lined  with  mucous  membrane.  This 
rests  on  some  fibrous  tissue,  outside  of  which  is  a  thick  coat 
of  muscular  tissue,  striated  in  the  upper  third  of  the  tube, 
unstriated  lower  down  next  to  the  stomach.  This  is  arranged 
in  two  layers,  an  outer  layer  in  which  the  fibres  run  parallel 
to  the  long  axis  of  the  tube  ;  an  inner  layer  in  which  the 
fibres  are  wrapped  round  the  tube. 

After  food  has  been  thrust  into  the  oesophagus  by  the 
action  of  the  pharynx,  a  wave-like  contraction,  called  peri- 
staltic action,  of  the  muscular  wall  of  the  oesophagus  follows 
the  bolus  and  finally  thrusts  it  into  the  stomach. 

Drink  is  taken  in  exactly  the  same  way  as  food.  It  does 
not  fall  down  the  pharynx  and  gullet,  but  each  gulp  is 
grasped  and  passed  down.  Hence  it  is  that  jugglers  are 
able  to  drink  standing  upon  their  heads,  and  that  a  horse, 
or  ox,  drinks  with  its  throat  lower  than  its  stomach,  feats 
which  would  be  impossible  if  fluid  simply  fell  down  the 
gullet  into  the  gastric  cavity. 

During  these  processes  of  mastication,  insalivation,  and 
deglutition,  what  happens  to  the  food  is,  first,  that  it  is 
reduced  to  a  coarser  or  finer  pulp ;  secondly,  that  any 
matters  it  carries  in  solution  are  still  more  diluted  by  the 
water  of  the  saliva ;  thirdly,  that  any  starch  it  may  con- 
tain begins  to  be  changed  into  sugar  by  the  saliva,  whose 
formation  and  action  we  must  next  consider. 

8.  The  Salivary  Glands.  —  The  mucous  membrane 
which  lines  the  mouth  and  the  pharynx  is  beset  with 
minute  glands,  the  buccal  glands ;  but  the  great  glands 
from  which  the  cavity  of  the  mouth  receives  its  chief 
secretion  are  the  three  pairs  which  are  called  the  parotid, 
submaxillary,  sublingual    (Fig.  79). 

Each  parotid  gland  is  placed  just  in  front  of  the  ear,  and 
its  duct  passes  forwards  along  the  cheek,  until  it  opens  in 


THE    SALIVARY   GLANDS 


263 


the  interior  of  the  mouth,  opposite  the  second  upper  grind- 
ing tooth. 

The  submaxillary  and  sublingual  glands  lie  between  the 
lower  jaw  and  the  floor  of  the  mouth,  the  submaxillary 
being  situated  farther  back  than  the  sublingual.  Their 
ducts  open  in  the  floor  of  the  mouth  below  the  tip  of  the 
tongue.  The  secretion  of  these  salivary  glands,  mixed  with 
that  of  the  small  glands  of  the  mouth,  constitutes  the  saliva. 


Fig. 


79- 


A  dissection  of  the  right  side  of  the  face,  showing  a,  the  sublingual,  b,  the  sub- 
maxillary glands,  with  their  ducts  opening  beneath  the  tongue  in  the  floor  of  the 
mouth  at  d;  c,  the  parotid  gland  and  its  duct,  which  opens  on  the  side  of  the  cheek 
at  e . 


The  salivary  glands  are  of  the  type  shown  in  Fig.  57,  6. 

Their  essential  part  consists  of  the  secreting  cells  which 
line  the  dilated  ends,  or  alveoli,  of  the  finest  branches  of 
their  ducts.  In  a  gland  which  is  resting,  that  is,  has  not 
been  secreting  for  some  time,  the  cells  are  large  and  nearly 
fill  the  alveoli  (Figs.  So  and  81,  A).  Each  cell  has  a 
nucleus  placed  either  near  its  outer  end  (many  of  the  sub- 
maxillary alveoli),  or  in  the  middle  of  the  cell  (parotid). 

The  protoplasm  of  the  body  of  the  cell  is  more  or  less 


264 


ELEMENTARY   PHYSIOLOGY 


completely  filled  up  with  granules,  which  are  better  seen  in 
pieces  of  the  fresh  gland  than  in  preserved  specimens. 

After  the  glands  have  been  secreting  for  some  ti?ne,  as  the 
result  either  of  taking  food  or  of  stimulating  the  nerves 
supplied  to  them,  the  appearance  of  their  cells  is  greatly 
changed  (Figs.  80,  B,  and  81,  C).  The  cells  are  now 
smaller ;  the  nucleus  has  become  more  distinct  and  in  the 
submaxillary  cells  has  moved  nearer  the  centre  of  the  cell ; 
the  granules  are  fewer  and  now  lie  near  the  inner  or  alveo- 
lar end  of  the  cells;- and  the  protoplasm,  being  freed  from 


a 

Fig.  80.  —  Sections  of  the  Submaxillary  Gland  hardened  and  stained. 
A,  after  rest;  B,  after  secretory  activity;  a,  a,  so-called  marginal  cells. 


granules,  is  now  much  more  distinct.  Between  these  two 
extremes  there  is  an  intermediate  stage  shown  in  Fig.  81,  B. 
The  differences  in  the  size  and  appearance  of  the  cells 
after  rest  and  after  activity  seem  to  show  quite  clearly  that, 
while  at  rest,  the  cells  build  up  material  which  is  stored  in 
their  substance,  and  hence  they  are  large.  In  the  submax- 
illary and  the  sublingual  glands  this  substance  is  largely 
mucinous,  in  the  parotid  albuminous,  and  it  is  deposited  as 


SALIVA   AND   ITS   SECRETION 


265 


separate  distinct  granules  in  the  body  of  the  cell.  Further,  it 
appears  that  during  their  activity  both  glands  discharge  their 
store  of  material  into  the  duct  leading  from  them,  and  hence 
the  cells  become  smaller  and  more  obviously  protoplasmic. 


Fig.  81.  —  Changes  in  the  Parotid  Gland  during  Secreting  Activity  (fresh). 
(Slightly  diagrammatic.) 

A,  after  rest;   B,  after  slight  activity;   C,  after  greater  activity. 


9.  Saliva  and  its  Secretion.  —  The  mixed  saliva  from  the 
several  glands  consists  chiefly  of  water,  holding  in  solution  a 
small  amount  of  proteid  matter,  some  inorganic  salts,  to 
which  its  faintly  alkaline  reaction  is  due,  a  small  amount  of 
mucin,  which  gives  to  saliva  its  well-known  sliminess,  and 
a  small  quantity  of  a  peculiar  substance  called  ptyaliii,  to 
which  the  digestive  power  of  the  liquid  is  due. 

Ordinarily  saliva  is  secreted  in  increased  quantity  as  soon 
as  food  is  introduced  into  the  mouth.  This  result  is  brought 
about  reflexly.  The  food  stimulates  the  ends  of  certain 
nerves  (Vth  and,  IXth  cranial,  see  p.  537)  which  supply  the 
walls  of  the  inside  of  the  mouth.  Impulses  pass  up  these 
nerves  to  the  brain,  and  from  this  organ  other  impulses 
pass  down  to  the  glands  and  make  their  cells  secrete. 

Some  of  the  experimental  evidence  that  the  salivary 
glands  are  under  nervous  control  is  as  follows  :  — 

The  submaxillary  gland  is  supplied  by  a  nerve  which  is  a 
branch  of  the  Vllth  cranial  nerve  (see  p.  537),  and  which, 


266  ELEMENTARY   PHYSIOLOGY  less. 

since  it  crosses  the  tympanic  cavity  or  drum  of  the  ear 
(see  p.  406),  is  called  the  chorda  tympani  nerve.  When  this 
nerve  is  stimulated  three  things  happen  :  the  arteries  which 
supply  the  gland  with  blood  dilate,  and  there  is  a  very 
largely  increased  flow  of  blood  through  the  gland  ;  the  gland 
begins  to  pour  out  its  secretion  ;  and  the  cells  of  the  gland 
slowly  change  their  size  and  appearance  as  already  described. 
These  changes  show  that  a  good  deal  of  the  material  with 
which  the  cells  were  loaded  during  rest  has  been  discharged. 
The  granules  in  the  cells  are  the  immediate  forerunners  of 
the  organic  constituents  of  the  saliva,  the  proteids,  the 
mucin,  and  the  ptyalin,  and  undergo  final  chemical  trans- 
formation into  these  constituents  at  the  time  of  discharge. 
But  at  the  same  time  the  cells  have  discharged  a  large 
quantity  of  water  and  some  salts,  and  the  water  and  salts 
can  have  come  only  from  the  blood.  The  question  at  once 
arises  :  has  the  increased  supply  of  blood  simply  led  to  an 
increased  flow  of  water  and  salts  through  the  cells,  which 
has  carried  away  with  it  the  accumulated  materials  of  the 
cell-substance,  the  whole  process  being  largely  filtrational ; 
or  has  the  stimulation  of  the  nerve  not  only  made  the  cells 
discharge  some  of  their  substance,  but  also  made  them  take  up 
water  and  salts  from  the  blood  and  pass  these  as  well  through 
the  cells?  The  evidence  in  support  of  the  latter  mode  of 
action  seems  conclusive.  For,  first,  an  increased  tempo- 
rary secretion  may  be  observed  on  stimulating  the  nerve  even 
after  the  blood-supply  to  the  gland  has  been  cut  off;  and, 
secondly,  if  certain  drugs,  such  as  atropine,  be  injected  into 
the  animal,  then,  although  the  arteries  dilate  to  the  full 
extent  when  the  nerve  is  stimulated,  no  increased  secretion 
takes  place.  Evidently,  when  the  gland  secretes  it  is 
because  the  impulses  which  ?-each  it  along  the  nerve  exert 
a  direct  influence  on  its  cells.      These  impulses   make  the 


vil  SOLUBLE   FERMENTS   OR   ENZYMES  267 

cells  take  up  water  and  salts  and  discharge  them,  together 
with  the  stored  cell-substance,  as  saliva  into  the  ducts. 
The  increased  blood- supply,  while  not  causing  the  secre- 
tion, is  necessary  if  the  cells  are  to  continue  to  secrete,  for 
it  is  from  the  blood  alone  that  they  can  obtain  all  that  they 
require  for  the  manufacture  of  the  saliva. 

10.  The  Action  of  Saliva.  —  Saliva  does  not  act  on 
proteids  or  fats,  but,  if  a  little  of  it  be  mixed  with  ordinary 
starch- paste  and  warmed  to  the  temperature  of  the  body,  by 
means  of  its  ptyalin  it  turns  that  starch  into  sugar.  This 
sugar  is  identical  with  that  obtained  from  malt  in  brewing, 
and  is  hence  known  as  maltose.  Although  this  chemical 
change  is,  without  doubt,  of  some  use  to  the  body,  its 
importance  must  not  be  over-estimated.  For  in  many 
animals  the  action  of  their  saliva  on  starch  is  very  slight, 
and,  moreover  (see  p.  287),  the  larger  part  of  the  starch  we 
eat  is  digested,  that  is,  changed  into  a  sugar,  while  the  food  is 
in  the  intestine  and  under  the  action  of  the  pancreatic  juice. 
The  chief  use  of  the  saliva  is  mechanical  rather  than  chem- 
ical, inasmuch  as  it  moistens  the  food  and  thereby  assists 
mastication  and  makes  the  swallowing  of  the  food  easy. 

11.  Soluble  Ferments  or  Enzymes. — The  peculiar  sub- 
stance, ptyalin,  to  which  the  chemical  action  of  saliva  on 
starch  is  due,  belongs  to  a  class  of  substances  known  as 
soluble  ferments  or  enzymes.  The  word  ferment  was  origi- 
nally applied  to  a  living  organism  such  as  yeast,  which,  as 
in  brewing,  while  converting  sugar  into  alcohol,  causes  at 
the  same  time,  on  account  of  the  simultaneous  production 
of  carbonic  acid  gas,  a  boiling  up  or  frothing  of  the  liquor  ; 
hence  the  name  "ferment "  {Jervere  =  to  boil  up). 

But  it  is  known  now  that  such  organised  ferments  can 
be  made  to  yield  extracts  which  may  be  filtered  so  as  to 
be  quite  free  from  organisms  and  still  be  able  to  produce 


268  ELEMENTARY   PHYSIOLOGY  less. 

the  same  changes  as  did  the  cells  from  which  they  are 
prepared.  Hence  the  name  of  soluble  ferment  or  enzyme 
(£vix-q  =  yeast)  was  given  to  the  substance  in  solution  which 
can  bring  about  the  same  changes  as  the  parent  cell. 

Very  little  is  known  of  the  chemical  nature  of  enzymes, 
but  they  are  strongly  characterised  by  certain  facts  as  to 
the  conditions  under  which  their  action  takes  place.  Thus  : 
(i)  Very  minute  quantities  will  effect  a  change  in  a  mass 
of  the  substance  on  which  they  are  working,  which  is  enor- 
mously large  compared  with  the  minute  mass  of  the  enzyme. 
(ii)  Their  action  depends  closely  on  temperature.  At 
o°  C.  (320  F.)  they  cease  to  act;  as  the  temperature  rises 
they  become  increasingly  active,  and  are  most  active  at  about 
400  C.  (1040  F.).  At  higher  temperatures  they  become  less 
active  and  lose  their  powers  permanently  if  once  heated  to 
ioo°  C.  (2 1 20  F.),  as  by  boiling:  they  are  then  said  to  be 
"  killed."  (hi)  Their  action  in  many  cases  depends  on  the 
reaction,  whether  acid  or  alkaline  or  neutral,  of  the  solution 
in  which  they  are  at  work.1  (iv)  Their  action  stops  in  pres- 
ence of  an  excess  of  the  special  products  of  their  activity. 
And  (v)  it  has  not  so  far  been  conclusively  proved  that  the 
enzymes  are  themselves  used  up  during  the  changes  which 
they  produce  on  other  substances. 

Nearly  all  the  chemical  changes  which  the  food  under- 
goes in  the  alimentary  canal  are  brought  about  by  the  action 
of  these  soluble  ferments  or  enzymes. 

12.  The  Structure  of  the  Stomach. — The  stomach,  like 
the  gullet,  consists  of  a  tube  with  muscular  walls  lined  by 
mucous  membrane  and  covered  by  peritoneum ;  but  it 
differs  from  the  gullet  in  several  circumstances.  In  the  first 
place,  its  cavity  is  much  larger,  and  its  left  end  is  produced 

1  Thus,  the  pepsin  of  gastric  juice  acts  best  in  presence  of  hydrochloric 
acid  and  the  trypsin  of  pancreatic  juice  in  presence  of  sodium  carbonate. 


THE   STRUCTURE   OF  THE   STOMACH 


269 


into  an  enlargement,  which,  because  it  is  on  the  heart  side 
of  the  body,  is  called  the  cardiac  part  (Fig.  82,  b).  The 
opening  of  the  gullet  into  the  stomach,  termed  the  cardiac 
aperture,  is  consequently  nearly  in  the  middle  of  the  whole 
length  of  the  organ,  which  presents  a  long,  convex,  greater 
curvature,  along  its  front  or  under  edge,  and  a  short,  con- 
cave, lesser  curvature,  on  its  back  or  upper  contour. 
Towards    its   right    extremity   the    stomach   narrows,   and, 


Fig.  S2.  —  The  Stomach  laid  open. 

a,  the  oesophagus;  b,  the  cardiac  dilatation ;  c,  the  lesser  curvature;  d,  the  pylorus; 
e,  the  biliary  duct;_/",  the  gall-bladder;  g,  the  pancreatic  duct  opening  in  common 
with  the  cystic  duct  opposite  h  /  h,  i,  the  duodenum. 


where  it  passes  into  the  intestine,  the  muscular  fibres  are 
so  disposed  as  to  form  a  sort  of  sphincter  around  the  aper- 
ture of  communication.  This  constriction  is  called  the 
pylorus  (Fig.  82,  d). 

The  muscular  coat  of  the  stomach,  consisting  of  unstriated 
muscular  tissue,  is  made  up  of  two  chief  layers,  an  outer 
longitudinal  and  an  inner  circular,  together  with  an  incom- 


270  ELEMENTARY   PHYSIOLOGY  .    less. 

plete  layer  of  muscle  fibres  which  are  continuous  with  the 
circular  fibres  of  the  oesophagus,  and  which,  running  ob- 
liquely, merge  into  the  internal  circular  layer  of  the  stomach. 
The  mucous  membrane  which  lines  the  stomach  is  loosely 
attached  to  the  muscular  coat  by  a  layer  of  areolar  connec- 
tive tissue.  This  is  called  the  submucous  coat,  and  it  is  in 
this  layer  that  the  nerves,  blood-vessels,  and  lymphatics  run 
for  the  supply  of  the  mucous  membrane. 

The  mucous  membrane  lining  the  wall  of  the  stomach 
contains,  or  rather  is  made  up  of,  a  multitude  of  small 
glands,  the  gastric  glands,  packed  closely  side  by  side  with 
delicate  adenoid  tissue  between  them,  and  opening  upon 
the  inner  surface  of  the  stomach.  These  are  on  the  whole 
simple  in  nature,  being  long,  tubular  glands,  but  they  vary 
in  character,  their  blind  ends  being  more  divided  and 
twisted  at  one  part  of  the  stomach  than  another. 

Each  gland  is  lined  by  cells  which  at  the  mouth  of  the 
gland  are  columnar  and  secrete  mucin ;  but  deeper  down 
in  the  tubes  they  are  cubical  and  granular.  These  are  the 
central  cells  (Fig.  83,  c) .  A  second  kind  of  cell  may  also 
be  seen  lying  scattered  irregularly  between  the  basement 
membrane  of  the  gland  and  its  central  cells  :  these  are  the 
parietal  cells  (Fig.  83,/).  Oval  in  shape,  they  have  a  well- 
defined  outline  and  their  cell-substance  is  usually  very  finely 
granular.  The  glands  near  the  pyloric  end  of  the  stomach 
differ  from  those  of  the  rest  of  the  mucous  membrane,  chiefly 
and  essentially  by  not  containing  any  of  these  parietal  cells. 

13.  Gastric  Juice  and  its  Secretion.  —  The  liquid  secreted 
by  the  glands  of  the  stomach  is  called  gastric  juice.  Pure 
gastric  juice  is  a  clear,  acid  fluid  and  consists  of  little  more 
than  water  containing  a  few  saline  matters  in  solution ;  its 
acidity  is  due  to  the  presence  of  free  hydrochloric  acid  to 
the  extent  of  .2  per  cent.     It  possesses,  however,  in  addi- 


THE   ACTION   OF   GASTRIC   JUICE 


271 


tion  a  small  quantity  of  a  peculiar  substance  called  pepsin, 
a  soluble  ferment  or  enzyme  in  many  respects  similar  to, 
though  very  different  in  its  effects  from,  ptyalin,  and  also 
a  similar  ferment  called  reixnin. 

When  the  stomach  is  empty,  its  mucous  membrane  is 
pale  and  hardly  more  than  moist.  Its  small  arteries  are 
then  in  a  state  of  constriction,  and  com- 
paratively little  blood  is  sent  through  it. 
On  the  entrance  of  food  a  vaso- motor 
action  is  set  up,  which  causes  these  small 
arteries  to  dilate  ;  the  mucous  membrane 
consequently  receives  a  much  larger 
quantity  of  blood,  and  it  becomes  very 
red.  At  the  same  time  the  cells  of  the 
glands  begin  to  form  their  secretion. 
The  whole  process  is  exactly  similar  in 
principle  to  that  already  described  in 
the  case  of  the  secretory  activity  of  the 
submaxillary  gland  (p.  265).  The  gran- 
ules of  the  central  cells  of  the  glands 
gradually  disappear  and  are  believed  to 
be  transformed  into  pepsin  and  perhaps 
rennin.  It  has  been  thought,  but  it  is 
not  definitely  established,  that  the  parietal 
cells  produce  the  hydrochloric  acid  of 
the  juice.  The  water  and  salts  come 
directly  from  the  blood. 

14.    The  Action  of  Gastric  Juice. — 
It  is  easy  to  ascertain  the  properties  of 
gastric  juice  experimentally,  by  putting  a  small  portion  of 
the  mucous  membrane  of  a  stomach  into  water  made  acid 
by  the  addition  of  .2-5  per  cent,  of  hydrochloric  acid  and 
containing  small  pieces  of  meat,  hard-boiled  egg,  or  other 


Fig.  83. — One  of  the 
Glands  which  se- 
crete Gastric  Juice. 

D,  the  duct  or  mouth 
of  the  gland",  m,  mucous 
cells  lining  the  mouth  of 
the  gland  and  covering 
the  inner  surface  of  the 
mucous  membrane:  c, 
central   cells;  p,   parietal 


272  ELEMENTARY   PHYSIOLOGY  less. 

proteids,  and  keeping  the  mixture  at  a  temperature  of  abou 
400  C.  (1040  F.).  After  a  few  hours  it  will  be  found  that 
the  white  of  egg,  if  not  in  too  great  quantity,  has  become 
dissolved  :  while  all  that  remains  of  the  meat  is  a  pulp,  con- 
sisting chiefly  of  the  connective  tissue  and  fatty  matters 
which  it  contained.  This  is  artificial  digestion,  and  it  has 
been  proved  by  experiment  that  precisely  the  same  opera- 
tion takes  place  when  food  undergoes  natural  digestion 
within  the  stomach  of  a  living  animal. 

The  solvent  power  of  gastric  juice  over  proteids  is  due 
to  the  pepsin ;  gastric  juice  which  has  been  boiled,  in  which 
case  all  the  ferment  it  contains  is  "  killed"  (see  p.  268),  is 
quite  inactive  although  it  contains  the  usual  amount  of  acid. 

The  characteristic  proteid  which  is  formed  during  the 
solvent  action  of  the  juice  is  called  peptone,  and  has  pretty 
much  the  same  characters  whatever  the  nature  of  the  pro- 
teid which  has  been  digested. 

Peptone  differs  from  all  other  proteids  in  its  extreme 
solubility,  and  characteristically  in  the  fact  that  it  is  highly 
diffusible,  and  hence  in  the  readiness  with  which  it  passes 
through  animal  membranes.  Many  proteids,  as  fibrin,  are 
naturally  insoluble  in  water,  and  others,  such  as  white  of 
egg,  though  apparently  soluble,  are  not  completely  so,  and 
can  be  rendered  quite  solid  or  coagulated  by  being  simply 
heated,  as  when  an  egg  is  boiled.  A  solution  of  peptone, 
however,  is  perfectly  fluid,  does  not  become  solid,  and  is 
not  at  all  coagulated  by  boiling.  Again,  if  a  quantity  of 
albumin,  such  as  white  of  egg  or  serum  of  blood,  be  tied 
up  in  a  bladder,  and  the  bladder  immersed  in  water,  very 
little  if  any  of  the  proteid  will  pass  through  the  bladder  into 
the  water,  provided  that  there  are  no  holes.1     If,  however, 

l  This  experiment  may  be  readily  made  with  the  apparatus  shown  in 
Fig.  45,  p.  145. 


vii  THE  ACTION   OF   GASTRIC  JUICE  27^ 

peptone  be  used  instead  of  albumin,  a  very  large  quantity 
will  speedily  pass  through  into  the  water,  and  a  quantity  of 
water  will  pass  from  the  outside  into  the  bladder,  causing  it 
to  swell  up.  This  diffusive  passage  of  a  substance  through 
a  membrane  is  called  osmosis,  and  is  evidently  of  great 
importance  in  the  economy ;  and  the  purpose  of  the  con- 
version of  the  various  proteids  by  digestion  into  peptone 
seems  to  be,  in  part  at  least,  to  enable  this  class  of  food- 
stuff to  pass  readily  into  the  blood  through  the  thin  partition 
formed  by  the  walls  of  the  mucous  membrane  of  the  intes- 
tine and  the  coats  of  the  capillaries.  Similarly,  starch,  even 
when  boiled,  and  so  partially  dissolved,  is  not  diffusible  and 
will  not  pass  through  membranes,  whereas  sugar  does  so 
with  the  greatest  ease.  Hence  the  reason  of  the  conversion 
of  starch,  by  digestion,  into  sugar. 

The  rennin  of  gastric  juice  causes  the  casein  in  milk  to 
clot  in  a  way  very  similar  to  that  in  which  fibrin-ferment 
gives  rise  to  a  clot  of  fibrin  by  its  action  on  fibrinogen 
(p.  140).  This  action  of  rennin  is  the  basis  of  cheese- 
making,  and  the  "rennet"  used  for  obtaining  the  curd  in 
the  latter  process  is  really  an  extract  of  the  mucous  mem- 
brane of  the  stomach  of  a  calf,  in  which  the  ferment  is 
peculiarly  plentiful. 

As  far  as  we  know,  gastric  juice  has  no  direct  action  on 
fats  ;  by  breaking  up,  however,  the  proteid  framework  of  the 
cells  in  which  animal  and  vegetable  fats  are  imbedded,  it 
sets  these  free,  and  so  helps  their  digestion  by  exposing 
them  to  the  action  of  other  agents.  It  appears  too,  that 
gastric  juice  has  no  direct  action  on  carbohydrates ;  on  the 
contrary  the  conversion  of  the  starch  into  sugar  begun  in 
the  mouth  appears  to  be  wholly  or  partially  arrested  by 
the  acidity  of  the  contents  of  the  stomach,  ptyalin  being 
active  only  in  an  alkaline  or  neutral  mixture. 

T 


274  ELEMENTARY   PHYSIOLOGY  less, 

By  continual  rolling  about,  with  constant  additions  of 
gastric  juice,  the  food  becomes  reduced  to  the  consistence 
of  pea-soup,  and  is  called  chyme.  In  this  state,  the  larger 
part  is  allowed  to  escape  through  the  pylorus  and  to  enter 
the  duodenum ;  but  a  very  small  portion  of  the  fluid  (con- 
sisting of  peptone  together  with  any  sugar  resulting  from  the 
partial  conversion  of  starch,  or  otherwise)  may  be  at  once 
absorbed,  making  its  way,  by  imbibition,  through  the  walls 
of  the  delicate  and  numerous  vessels  of  the  stomach  into 
the  current  of  the  blood,  which  is  rushing  through  the  gastric 
veins  to  the  portal  vein. 

15.  The  General  Arrangement  and  Structure  of  the 
Intestines. — The  intestines  (Figs.  84  and  86)  form  one 
long  tube,  with  mucous  and  muscular  coats,  like  the  stom- 
ach ;  and,  like  it,  they  are  enveloped  in  peritoneum.  They 
are  divided  into  two  portions  —  the  small  intestine  and  the 
large  intestine  ;  the  latter,  though  shorter,  having  a  much 
greater  diameter  than  the  former.  The  name  of  duodenum 
is  given  to  that  part  of  the  small  intestine,  about  ten  inches 
in  length,  which  immediately  succeeds  the  stomach.  It  is 
bent  upon  itself  and  fastened  by  the  peritoneum  against  the 
back  wall  of  the  abdomen,  in  the  loop  shown  in  Fig.  82,  h,  i. 
It  is  in  this  loop  that  the  head  of  the  pancreas  lies  (Fig.  75). 

The  rest  of  the  small  intestine,  of  which  the  part  next 
the  duodenum  is  called  the  jejunum  and  the  rest  the  ileum,  is 
no  wider  than  the  duodenum,  so  that  the  transition  from  the 
small  intestine  to  the  large  (Figs.  85,  a,  k,  and  &6,Il,ccec)  is 
quite  sudden.  The  opening  of  the  small  intestine  into 
the  large  is  provided  with  prominent  lips  which  project 
into  the  cavity  of  the  latter,  and  oppose  the  passage  of 
matters  from  it  into  the  small  intestine,  while  they  readily 
allow  of  a  passage  the  other  way.  This  is  the  ileo-ceecal 
valve  (Fig.  85,  d). 


THE   INTESTINES 


Fig.  84.  —  The  Viscera  of  a  Rabbit  as  seen  upon  simply  opening  the  Cavi- 
ties of  the  Thorax  and  Abdomen  without  any  further  Dissection. 

A,  cavity  of  the  thorax,  pleural  cavity  on  either  side;  B,  diaphragm;  C,  ven- 
tricles of  the  heart;  D,  auricles;  E,  pulmonary  artery;  F,  aorta;  G,  lungs  collapsed, 
and  occupying  only  the  back  part  of  chest;  H,  lateral  portions  of  pleural  membranes; 
/,  cartilage  at  the  end  of  sternum  (ensiform  cartilage) ;  A",  portion  of  the  wall  of  body 
left  between  thorax  and  abdomen;  a,  cut  ends  of  the  ribs;  L,  the  liver,  in  this  case 
lying  more  to  the  left  than  the  right  of  the  body;  M,  the  stomach,  a  large  part  of  the 
greater  curvature  being  shown;  iV,  duodenum  ;  O,  other  portions  of  the  small  intes- 
tine; P,  the  caecum,  so  largely  developed  in  this  and  other  herbivorous  animals;  Q. 
the  large  intestine. 


276  ELEMENTARY   PHYSIOLOGY  less. 

The  large  intestine  forms  a  blind  dilatation  beyond  the 
ileo-csecal  valve,  which  is  called  the  caecum  (Figs.  85,  k, 
and  86,  ccec)  ;  and  from  this  an  elongated  blind  process 
is  given  off,  which,  from  its  shape,  is  called  the  vermiform 
appendix  of  the  caecum   (Figs.  85,  b,  and  86,  verm). 


Fig.   85. 

The  junction  of  the  ileum,  a,  with  the  caecum,  k,  and  the  continuation  of  the  latter 
into  the  colon,  e;  d,  the  ileo-cascal  valve;  c,  the  opening  of  the  vermiform  appendix 
(3)  into  the  caecum. 

The  caecum  lies  in  the  lower  part  of  the  right  side  of  the 
abdominal  cavity.  The  colon  (Fig.  86),  or  first  part  of  the 
large  intestine,  passes  upwards  from  it  as  the  ascending 
colon ;  then  making  a  sudden  turn  at  a  right  angle,  it 
passes  across  to  the  left  side  of  the  body,  being  called  the 
transverse  colon  in  this  part  of  its  course  ;  and  next, 
suddenly  bending  backwards  along  the  left  side  of  the 
abdomen,  it  becomes  the  descending  colon.  This  reaches 
the  middle  line  and  becomes  the  rectum,  which  is  that  part 
of  the  large  intestine  which  opens  externally.  The  external 
opening  is  called  the  anus. 

The  intestines  are  slung  from  the  middle  line,  along 
the  vertebral  column,  of  the  abdominal  cavity  by  a  thin 
membrane   known  as  the  mesentery  (Fig.  87).     This  is  a 


THE   INTESTINES 


277 


continuation  of  the  peritoneum,  the  serous  membrane  that 
lines  the  whole  cavity  of  the  abdomen.  The  mesentery 
consists  really  of  two  layers,  between  which  the  nerves, 
blood-vessels,'  and  lymphatics  lie  which  supply  the  intestines. 


Asol 


verm 


Fig.  86.  —  The  Alimentary  Canal  in  the  Abdomen. 

R_,  right:  L,  left;  tz,  oesophagus;  st,  stomach;  py,  pylorus;  duo,  duodenum; 
JeJ<  jejunum;  //,  ileum;  ccec,  caecum;  A.col,  ascending  colon;  T.col,  transverse 
colon;   D.col,  descending  colon;  R,  rectum;  zierm,  vermiform  appendix. 

The  latter  thus  lie  in  a  fold  of  the  peritoneum,  somewhat  as 
a  man  lies  when  slung  in  a  hammock. 

Other  folds  of  the  peritoneum  similarly  support  the 
Other  organs  in  the  abdomen.     The  peritoneum  is  thus  a 


i78 


ELEMENTARY   PHYSIOLOGY 


double  bag  whose  relation  to  the  wall  of  the  abdomen  and 
to  the  organs  in  it  is  similar  to  that  of  the  pleurae  to  the 
walls  of  the  thorax  and  the  lungs. 

The  intestines  receive  their  blood  almost  directly  from 
the  aorta.  Their  veins  carry  the  blood  which  has  traversed 
the  intestinal  capillaries  to  the  portal  vein. 

The  intestines,  like  the  stomach,  are  made  up  of  four 
coats  :  the  external  peritoneum,  then  a  muscular  coat  con- 
nected by  a  submucous  layer  with  the  inner  or  mucous  coat. 


IV  Tt 


d.m. 


j>cn't  ___ 


b.v. 


rn.es : 


incest 


perit 


Fig.  87. 


■Diagram  to  show  how  the  Wall  of  the  Abdomen  is  made  up, 
and  how  the  mesentery  supports  the  intestine. 


The  body  is  supposed  to  be  cut  across,  and  the  intestine  is  represented  as  the 
section  of  a  straight  tube.  In  reality  the  space  between  the  intestine  and  the  body 
wall  is  filled  by  the  coils  of  the  intestine  and  by  other  organs. 

vert,  vertebra;  d.m,  muscles  of  back;  sk,  skin;  m1,  m~,  nfi,  the  three  muscle 
layers;  perit,  peritoneum;  no,  mesentery;   latest,  intestine;  b.v,  blood-vessels. 


The  muscular  coat  of  the  small  intestine  is  made  up  of 
two  layers  ;  an  outer  longitudinal,  an .  inner  circular.  As  in 
the  oesophagus,  the  circular  fibres  of  any  part  are  able  to 
contract  successively,  in  such  a  manner  that  the  upper 
fibres,  or   those  nearer   the  stomach,  contract   before    the 


vii  THE   INTESTINES  279 

lower  ones,  or  those  nearer  the  large  intestine.  It  follows 
from  this  peristaltic  contraction,  that  the  contents  of  the 
intestines  are  constantly  being  propelled,  by  successive  and 
progressive  narrowing  of  their  calibre,  from  their  upper 
towards  their  lower  parts.  And  the  same  peristaltic  move- 
ment goes  on  in  the  large  intestine  from  the  ileo-csecal 
valve  to  the  anus. 

The  submucous  layer  is  composed  of  loose  (areolar) 
connective  tissue,  and  carries  the  blood-vessels,  nerves, 
and  lymphatics. 

The  tube  of  mucous  membrane  which  forms  'the  inner 
coat  of  the  small  intestine  is  longer  than  the  muscular  tube 
which  surrounds  it ;  hence,  to  get  this  greater  length  of 
the  former  stowed  away  into  the  shorter  length  of  muscular 
tubing,  the  mucous  membrane  is  thrown  into  folds,  which 
must  evidently  lie  at  right  angles  to  its  long  axis.  These 
folds  serve  to  increase  the  surface  of  the  mucous  membrane 
and  are  called  valvulae  comiiventes. 

The  large  intestine  presents  noteworthy  peculiarities  in 
the  arrangement  of  the  longitudinal  muscular  fibres  of  the 
colon  into  three  bands,  which  are  shorter  than  the  walls  of 
the  intestine  itself,  so  that  the  latter  is  thrown  into  puckers 
and  pouches  (Fig.  84,  Q)  ;  these  are  known  as  the  sacculi, 
and  serve  for  the  same  purpose  as  the  valvules  conniventes 
of  the  small  intestine.  Moreover,  the  muscular  fibres  around 
the  anus  are  arranged  so  as  to  form  a  ring-like  sphincter 
muscle,  which  keeps  the  aperture  firmly  closed,  except 
when  defalcation  takes  place. 

The  mucous  membrane  of  both  small  and  large  intestine 
consists  largely  of  simple  tubular  glands  packed  side  by  side  ; 
they  are  known  as  the  glands  of  Lieberkuhn  (Fig.  88,  G.L.). 
Each  gland  is  lined  by  a  layer  of  columnar  cells  (Fig.  88,  C), 
among  which  occur  a  certain  number  of  mucous  cells.     The 


28o  ELEMENTARY   PHYSIOLOGY  less. 

glands  are  separated  from  one  another  by  adenoid  tissue 
(p.  1 1 6).  In  the  small  intestine  the  tissue  between  the 
mouths  of  the  glands  projects  into  the  cavity  of  the  intes- 
tine as  minute  club-shaped  processes,  the  villi,  which  are  set 
side  by  side  over  the  surface  of  the  mucous  membrane  like 
the  pile  on  velvet.     These  villi  are  absent  in  the  large  intestine. 

At  irregular  intervals  along  the  mucous  membrane  the 
lymphoid  tissue  between  the  glands  forms  small  rounded 
masses  crowded  with  leucocytes  like  lymphatic  glands,  and 
called  solitary  glands.  In  parts  of  the  small  intestine 
groups  of  these  follicles  are  found  packed  closely  together; 
they  are  then  known  as  Peyer's  patches. 

At  the  commencement  of  the  duodenum  are  certain 
small  racemose  glands,  called  the  glands  of  Brunner,  whose 
ducts  open  into  the  intestine.  Their  function  seems  to  be 
quite  unimportant. 

16.  The  Structure  of  the  Villi.  —  The  average  length  of 
a  villus  (Fig.  88,  A)  is  about  .5  -.7  of  a  millimetre  (-gL __L 
of  an  inch).  Running  up  its  centre  or  axis  is  a  relatively 
large  lymphatic  vessel,  which  ends  blindly  at  the  summit  of 
the  villus,  but  at  its  base  opens  into  the  lymphatics  in  the 
submucous  tissue.  This  central  lymphatic  is  called  a  lacteal. 
Lying  around  the  lacteal,  and  parallel  to  it,  are  a  few  small 
fibres  of  unstriated  muscle  derived  from  the  muscularis 
mucosae,  which  is  a  thin  layer  of  unstriated  muscle  in  the 
mucous  membrane,  lying  next  to  the  submucous  coat  (m.m.)  ; 
outside  these  again,  close  under  the  epithelium  of  the  villus, 
is  a  network  of  capillaries  {c),  which  receive  blood  from  an 
artery  in  the  submucous  layer  and  return  it  by  a  small  vein 
to  the  veins  of  the  same  layer. 

All  space  left  between  the  several  structures  so  far  de- 
scribed in  the  body  of  the  villus  is  filled  up  with  adenoid 
(lymphoid)   tissue,  which  is  continuous  with  that  between 


THE   STRUCTURE   OF  THE   VILLI 


281 


the  glands  of  Lieberkiihn,  and  whose  meshes  are  more  or 
less  crowded  with  leucocytes. 


sf —  m.m. 


Fig.  88. —  Diagram  of  Two  Villi  and  an  adjacent  Gland  of  Lieberki'hn 
(Hardy). 

A,  two  villi  with  a  gland  of  Lieberkiihn,  G.  L,  between  their  bases;  m.tn,  mus- 
cularis  mucosas;  /,  central  lacteal;  c,  blood-capillaries. 

B,  portion  of  epithelium  of  villus  more  highly  magnified  to  show  one  "  goblet  " 
cell  (above)  and  two  of  the  other  epithelial  cells;  C,  two  of  the  cells  which  line  the 
tube  of  the  gland  of  Lieberkiihn,  more  highly  magnified. 

The  epithelium  covering  a  villus  is  continuous  with  that 
lining  the  glands  of  Lieberkiihn  and  is  made  up  of  cells  of 
two  kinds  (Fig.  88,  B).     Of  these  the  large  majority  are 


2S2  ELEMENTARY   PHYSIOLOGY  less. 

tall,  columnar,  and  granular,  with  an  oval  nucleus.  The 
outer  end  of  each  cell  (on  the  surface  of  the  villus)  shows  a 
narrow,  strongly  striated  border.  These  cells  are  concerned 
in  the  absorption  of  digested  food.  Lying  between  these 
are  cells  which,  from  their  shape,  are  often  called  "  goblet " 
cells,  but  which  in  structure  are  practically  the  same  as  the 
mucous  cells  of  the  submaxillary  gland  already  described 
(p.  263).  These  cells  secrete  the  mucus  which  covers 
the  inside  of  the  intestine. 

17.  Succus  Entericus.  —  The  glands  of  Lieberkiihn  are 
supposed  to  form  a  secretion  known  as  succus  entericus, 
or  intestinal  juice,  which  they  then  discharge  into  the  intes- 
tine. The  precise  functions  of  this  secretion  are  not  wholly 
known  :  it  seems  to  be  able  to  convert  starch  and  various 
kinds  of  sugar  into  that  variety  of  sugar  known  as  dextrose. 
But,  on  the  whole,  it  probably  possesses  comparatively  little 
importance  as  a  digestive  agent. 

18.  The  Structure  of  the  Pancreas  and  its  Changes 
during  Secretion. — The  pancreas  is  a  racemose  gland,  but 
the  alveoli  in  which  the  ducts  end  are  somewhat  elongated 
as  compared  with  their  more  rounded  shape  in  the  salivary 
glands.  The  cells  in  each  alveolus  are  not  unlike  those  of 
the  parotid  gland  (p.  263).  When  the  gland  has  been  at 
rest  for  some  time  the  cells  are  large,  their  outlines  indistinct, 
and  they  are  thickly  loaded  except  at  their  outer  ends  with 
very  obvious  granules  (Fig.  89,  A.).  After  the  gland  has 
been  secreting  for  some  time,  the  cells  are  smaller,  their 
outline  distinct,  and  the  granules  have  largely  disappeared. 
Those  granules  which  remain  are  now  placed  at  the  inner 
ends  of  the  cells  next  to  the  lumen  of  the  alveolus  (Fig. 
89,  B.).  These  differences  in  the  appearance  of  the  cells  in 
the  two  conditions  of  rest  and  activity  show  quite  clearly 
that  while   at  rest  these  cells  build  up  material,  which  is 


vii      NATURE  AND   ACTION   OF   PANCREATIC  JUICE      2S3 

lodged  in  their  substance  as  obvious  granules,  and  discharge 
this  material  as  part  of  the  secretion  as  soon  as  they  become 
active.  Thus,  the  changes  taking  place  in  the  cells  of  the 
pancreas  during  secretion  are  essentially  the  same  as  those 
previously  described  in  the  case  of  the  salivary  glands,  and 
have  the  same  significance  in  explanation  of  the  phenomena 
of  secretion. 

19.  The  Nature  and  Action  of  Pancreatic  Juice. — 
Pancreatic  juice  is  a  somewhat  viscid  fluid,  alkaline  from 
the  presence  of  sodium  carbonate  and  containing  a  fairly 


B. 

Fig.  89.  —  A  Portion  of  the  Pancreas  of  a  Rabbit. 

A.  after  rest;  B.  after  activity. 

a,  granular  central  zone  of  the  cells;  b,  clear  outer  zone;  c,  lumen  of  alveolus; 
d,  junction  of  two  neighbouring  cells. 

large  amount  of  proteid  in  solution.  It  contains  further,  as 
its  most  important  constituents,  three  soluble  ferments.  Of 
these  one,  which  is  called  trypsin,  is  so  far  like  pepsin  that 
it  converts  proteids  into  peptones,1  but  it  differs  from  pepsin 
in  several  respects.  In  the  first  place  trypsin  is  most  active 
in  an  alkaline  solution,  such  as  of  1  per  cent,  sodium  car- 

1  An  artificial  pancreatic  digestion  of  proteids  may  be  carried  on  in  the 
way  already  described  for  pepsin  (p.  271),  using  as  a  digestive  fluid  a  1  per 
cent,  solution  of  sodium  carbonate  to  which  some  of  the  extract  of  pancreas 
sold  as  "  Liquor  pancreaticus  "  has  been  added. 


284  ELEMENTARY   PHYSIOLOGY  less. 

bonate,  while  pepsin  will  act  only  in  the  presence  of  an' 
acid.  In  the  next  place,  the  change  which  proteids  undergo 
by  the  action  of  trypsin  does  not  end  with  the  formation  of 
peptones,  as  it  does  in  the  case  of  pepsin,  but  proceeds 
further,  and  some  of  the  peptone  is  broken  down  into 
the  crystalline  substances  known  as  lencin  and  iyrosin. 
Of  these  the  leucin  is  peculiarly  interesting,  inasmuch  as 
after  it  is  absorbed  it  is  carried  to  the  liver  in  the  blood 
of  the  portal  vein  and  apparently  is  converted  by  the  liver 
into  urea  (see  p.  214). 

The  second  ferment  in  pancreatic  juice,  called  amylopsin, 
resembles  the  ptyalin  of  saliva  in  so  far  as  it  converts  starch 
into  sugar,  but  it  acts  more  energetically. 

The  third  ferment,  called  steapsin,  has  no  action  on 
either  proteids  or  carbohydrates,  but  it  acts  on  the  ordinary 
fats  in  such  a  way  as  to  split  them  into  glycerine  and  a  fatty 
acid.  The  latter  uniting  with  the  alkali  of  the  pancreatic 
juice  forms  soaps,  and  this  process  is  known  as  saponifica- 
tion. The  soaps  so  formed  are  important,  for  they  help 
greatly  in  reducing  the  rest  of  the  fats  to  that  state  of  fine 
subdivision,  known  as  an  emulsion,  which  is  an  important 
aid  to  further  saponification  and  ultimate  absorption. 

Pancreatic  juice,  as  containing  these  three  ferments,  acts, 
therefore,  on  all  three  classes  of  food-stuffs,  peptonising  the 
proteids,  saponifying  and  emulsifying  the  fats,  and  convert- 
ing starch  into  sugar. 

Although  the  most  obvious  function  of  the  pancreas  is  to 
secrete  a  digestive  juice,  there  are  reasons  for  supposing  that 
it  has  other  important  uses.  If  it  be  removed  from  an  animal, 
a  large  quantity  of  sugar  speedily  appears  in  the  urine  and 
the  animal  wastes  away.  Such  a  condition  is  not  infrequently 
observed  in  man,  where  it  is  known  as  diabetes ;  and  in  some 
cases  of  diabetes  the  pancreas  is  found  *o  be  diseased. 


vii  THE  FUNCTION  OF  BILE  285 

The  pancreas  thus  seems  to  exert  some  control  over  the 
nutrition  of  the  body,  probably  by  means  of  an  internal 
secretion,  in  a  way  somewhat  similar  to  (though  differing  in 
its  results  from)  the  influence  exerted  by  the  thyroid  gland 
and  the  suprarenal  bodies  (see  p.  247). 

20.  The  Function  of  Bile.  —  Bile  has  of  itself  no  direct 
chemical  action  on  food-stuffs,  but  as  an  alkaline  fluid, 
poured  into  the  intestine  in  large  quantity,  it  serves  to  neu- 
tralise the  acidity  of  the  chyme  as  the  latter  leaves  the 
stomach,  and  thus  prepares  it  for  the  action  of  pancreatic 
juice.  Further,  by  means  of  the  bile-salts  bile  plays  an 
important  part,  when  mixed  with  pancreatic  juice,  in  lead- 
ing to  the  emulsification  of  fats,  and  also  facilitates  their 
subsequent  absorption.  The  bile-pigments  and  cholesterin 
are  excretions. 

21.  The  Changes  undergone  by  Food  in  the  Intestines. 
—  The  only  secretions,  besides  those  of  the  proper  intestinal 
glands,  which  enter  the  intestine,  are  those  of  the  liver  and 
the  pancreas  —  the  bile  and  the  pancreatic  juice.  The  ducts 
of  these  organs  have  a  common  opening  in  the  middle  of 
the  bend  of  the  duodenum  ;  and,  since  the  common  duct 
passes  obliquely  through  the  coats  of  the  intestine,  its  walls 
serve  as  a  kind  of  valve,  obstructing  the  flow  of  the  con- 
tents of  the  duodenum  into  the  duct,  but  readily  permitting 
the  passage  of  bile  and  pancreatic  juice  into  the  duodenum 
(Figs.  75  and  82). 

After  gastric  digestion  has  been  going  on  some  time,  and 
the  semi-digested  food  begins  to  pass  on  into  the  duodenum, 
the  pancreas  comes  into  activity,  its  blood-vessels  dilate,  it 
becomes  red  and  full  of  blood,  its  cells  secrete  rapidly,  and 
a  copious  flow  of  pancreatic  juice  takes  place  along  its  duct 
into  the  intestine. 

The  secretion  of  bile  by  the  liver  is  much  more  continu- 


2S6  ELEMENTARY  PHYSIOLOGY  less. 

ous  than  that  of  the  pancreas,  and  is  not  so  markedly  in- 
creased by  the  presence  of  food  in  the  stomach.  There  is, 
however,  a  store  of  bile  laid  up  in  the  gall-bladder ;  and  as 
the  acid  chyme  passes  into  the  duodenum,  and  flows  over 
the  common  aperture  of  the  bile  and  pancreatic  ducts,  a 
quantity  of  bile  from  this  reservoir  in  the  gall-bladder  is 
ejected  into  the  intestine.  The  bile  and  pancreatic  juice- 
together  here  mix  with  the  chyme  and  produce  remarkable 
changes  in  it. 

In  the  first  place,  the  alkali  of  these  juices  neutralises  the 
acid  of  the  chyme  ;  in  the  second  place,  as  has  been  seen, 
both  the  bile  and  the  pancreatic  juice  exercise  an  emulsify- 
ing influence  over  the  fatty  matters  contained  in  the  chyme, 
and  this  action  is  specially  well  marked  when  bile  and  pan- 
creatic juice  are  mixed.  The  fat,  as  it  passes  from  the 
stomach,  is  very  imperfectly  mixed  with  the  other  constitu- 
ents of  the  chyme ;  and  the  drops  of  fat  or  oil  (for  all  the 
fat  of  the  food  is  melted  by  the  heat  of  the  stomach)  readily 
run  together  into  larger  masses.  By  the  combined  action, 
however,  of  the  bile  and  pancreatic  juice  the  large  drops  of 
fat  which  pass  into  the  intestine  from  the  stomach  are  broken 
up  into  exceedingly  minute  particles,  and  thoroughly  mixed 
with  the  rest  of  the  contents ;  they  are  brought  in  fact  to 
very  much  the  same  condition  as  that  in  which  fat  {i.e.  but- 
ter) exists  in  milk.  When  this  emulsifying  has  taken  place 
the  contents  of  the  small  intestine  no  longer  appear  grey, 
like  the  chyme  in  the  stomach,  but  white  and  milky  ;  in  fact, 
it  and  milk  are  white  for  the  same  reason,  viz.,  on  account 
of  the  multitude  of  minute  suspended  fatty  particles  reflect- 
ing a  great  amount  of  light. 

The  contents  of  the  small  intestine,  thus  white  and  milky, 
are  sometimes  called  chyle ;  but  it  is  best  to  reserve  this 
name  for  the  contents  of  the  lacteals,  of  which  we  shall  have 
to  speak  directly. 


<7I  CHANGES   UNDERGONE   BY   FOOD  287 

The  emulsifying  of  the  fats  is  not,  however,  the  only 
change  going  on  in  the  small  intestine.  For  this  is  simply 
preliminary  to  their  being  split  up,  by  the  pancreatic  juice, 
into  glycerine  and  fatty  acid,  and  the  subsequent  formation 
of  soaps.  It  seems  probable  that  much,  if  not  all,  of  the 
fat  thus  goes  to  form  soaps,  which  are  soluble  and  diffusible. 
Moreover  the  pancreatic  juice  has  an  action  on  starch  simi- 
lar to  that  of  saliva,  but  much  more  powerful.  During  the 
short  stay  in  the  mouth  very  little  starch  has  had  time  to  be 
converted  into  sugar,  and  in  the  stomach,  as  we  have  seen, 
the  action  of  the  saliva  is  arrested.  In  the  small  intestine, 
however,  the  pancreatic  juice  takes  up  the  work  again  ;  and, 
indeed,  by  far  the  greater  part  of  the  starch  which  we  eat  is 
digested,  that  is,  changed  into  sugar,  by  the  action  of  this 
juice. 

Nor  is  this  all,  for,-in  addition  to  the  above,  the  alkaline 
pancreatic  juice  has  a  powerful  effect  on  proteids  very  simi- 
lar to  that  exerted  by  the  acid  gastric  juice :  it  converts 
them  into  peptones,  and  the  peptones  so  produced  do  not 
differ  materially  from  the  peptones  resulting  from  gastric 
digestion.  At  the  same  time  a  variable  amount  of  leucin 
and  tyrosin  make  their  appearance  as  the  result  of  the 
further  action  of  pancreatic  juice  on  the  peptones. 

Hence  it  appears  that,  while  in  the  mouth  carbohydrates 
only,  and  in  the  stomach  proteids  only,  are  digested,  in  the 
intestine  all  three  kinds  of  food-stuffs,  proteids,  fats,  and 
carbohydrates,  are  either  completely  dissolved  and  made 
diffusible,  or  minutely  subdivided,  and  so  prepared  for  their 
passage  into  the  vessels. 

As  the  food  is  thrust  along  the  small  intestine  by  the 
grasping  action  of  the  peristaltic  contractions,  the  digested 
matter  which  it  contains  is  absorbed,  that  is,  passes  away 
from  the  interior  of  the  intestine  into  the  blood-vessels  and 


288  ELEMENTARY   PHYSIOLOGY  less. 

lacteals  lying  in  the  intestinal  walls.  So  that,  by  the  time 
the  contents  of  the  intestine  have  reached  the  ileo-caecal 
valve,  a  great  deal  of  the  nutritious  matter  has  been  re- 
moved. Still,  even  in  the  large  intestine,  some  nutritious 
matter  has  still  to  be  acted  upon  ;  and  we  find  that,  in  the 
caecum  and  commencement  of  the  large  intestine,  changes 
are  taking  place,  apparently  somewhat  of  the  nature  of  fer- 
mentation, whereby  the  contents  become  acid.  These 
changes  are  largely  the  result  of  the  activity  of  certain 
minute  organisms  or  organised  ferments  (bacteria,  etc.). 

One  marked  feature  of  the  changes  undergone  in  the 
large  intestine  is  the  rapid  absorption  of  water.  Whereas, 
in  the  small  intestine,  the  amount  of  fluid  secreted  into  the 
canal  about  equals  that  which  is  removed  by  absorption,  so 
that  the  contents  at  the  ileo-caecal  valve  are  about  as  fluid 
as  they  are  in  the  duodenum ;  in  the  large  intestine,  on  the 
contrary,  especially  in  its  later  portions,  the  contents  become 
less  and  less  fluid.  At  the  same  time  a  characteristic  odour 
and  colour  are  developed,  and  the  remains  of  the  food,  now 
consisting  either  of  undigestible  material,  or  of  material  which 
has  escaped  the  action  of  the  several  digestive  juices,  or  with- 
stood their  influence,  gradually  assume  the  characters  of 
faeces. 

22.  Absorption  from  the  Intestines.  —  A  great  deal  of 
the  absorption  takes  place  in  the  small  intestine  (though  the 
process  is  continued  in  the  large  intestine) ,  and  there  can 
be  no  doubt  that  it  is  largely  effected  by  means  of  the  villi. 
Each  villus,  as  we  have  seen  (p.  280),  is  covered  by  a  layer 
of  epithelium,  and  contains  in  the  centre  a  lacteal  radicle, 
between  which  and  the  epithelium  lies  a  network  of  capil- 
lary blood-vessels  imbedded  in  a  delicate  tissue.  Now  after 
a  meal  containing  fat  the  epithelium  cells  covering  the  villi 
are  loaded  with  minute  droplets  of  fat.     It  has  long  been 


vil  ABSORPTION   FROM   THE   INTESTINES  289 

supposed  that,  in  some  way  or  other  not  thoroughly  under- 
stood, the  majority  of  the  minute  particles  of  the  finely 
divided,  emulsified  fat  in  the  intestine  passed  into  the  cells 
of  the  epithelium  directly.  It  now  seems  more  probable 
that  the  fat  is  absorbed  in  the  form  of  dissolved  soap,  in 
the  same  manner  as  the  peptone  and  sugar  to  be  dis- 
cussed below,  and  after  entering  the  cells  is  changed  back 
into  fat  droplets.  However  this  may  be,  and  the  manner 
of  the  absorption  is  not  wholly  clear,  the  droplets  later  leave 
the  epithelial  cells,  and  go  past  the  capillary  blood-vessels, 
into  the  central  lacteal  radicle ;  so  that,  after  a  fatty  meal, 
these  lacteal  radicles  of  the  villi  become  filled  with  fat. 
The  lacteal  radicle  is  continuous  with  the  interior  of  the 
lymphatic  vessels  which  ramify  in  the  walls  of  the  intestine, 
and  which  pass  into  the  larger  lymphatic  vessels  running 
along  the  mesentery  towards  the  thoracic  duct.  Into  these 
vessels  the  finely  divided  fat  passes  from  the  lacteal  radicle 
of  the  villus,  and,  mixing  with  the  ordinary  lymph  contained 
in  these  vessels,  gives  their  contents  a  white,  milky  appear- 
ance. Lymph  thus  white  and  milky  from  the  admixture  of 
a  large  quantity  of  finely  divided  fat  is  called  chyle  ;  and 
this  white  chyle  may  after  a  meal  be  traced  along  the  lym- 
phatics of  the  mesentery  to  the  thoracic  duct,  and  along  the 
whole  course  of  that  vessel  to  its  junction  with  the  venous 
system.  After  a  meal,  in  fact,  this  vessel  is  continually  pour- 
ing into  the  blood  a  large  quantity  of  chyle,  i.e.  of  lymph 
made  white  and  milky  by  the  admixture  of  fats  drawn  from 
the  villi  of  the  small  intestine. 

In  the  case  of  the  proteids  and  carbohydrates,  the  result 
of  digestion  has  been  to  produce  a  solution  of  peptones  and 
sugars,  which  are  extremely  soluble  and  highly  diffusible. 
Now  we  know  that  if  such  a  solution  is  separated  by  a  thin 
membrane  from  a  solution  of  ordinary  non- diffusible  pro- 
it 


29o  ELEMENTARY   PHYSIOLOGY  less, 

teids,  there  will  be  a  rapid  transmission  of  the  diffusible 
substances  through  and  across,  the  membrane.  The  con- 
ditions necessary  for  such  a  process  are  evidently  present 
in  the  intestines,  where  the  solution  in  its  interior  is  sepa- 
rated by  what  is  practically  a  thin  membrane  from  the 
(albuminous)  blood  in  the  capillaries  just  below  the  epithe- 
lial cells.  It  is  thus  very  tempting  to  suppose  that  the  ab- 
sorption of  peptones  and  sugars  (also  of  salts)  is  the  result 
of  their  diffusibility  and  of  the  conditions  to  which  they  are 
exposed.  And  indeed  within  certain  limits  this  view  is  cor- 
rect. But  it  does  not  by  any  means  explain  the  whole  pro- 
cess. For  if  substances  of  differing  diffusibilities  be  placed 
in  the  intestines  it  is  not  found  that  the  most  diffusible  sub- 
stance is  necessarily  absorbed  the  fastest.  In  fact  we  find 
that  the  details  of  the  absorption  are  in  many  ways  so  pecul- 
iar that  we  must  look  to  the  living  epithelial  cells  of  the 
villi  as  determining  and  completely  controlling  the  process, 
which  is  thus  partly  physical  but  chiefly  due  to  the  special 
activity  of  cells. 

The  fats  pass,  as  already  stated,  into  the  lacteals  and 
thence  through  the  lymphatic  vessels  and  thoracic  duct  into 
the  blood.  Peptones  and  sugar,  on  the  other  hand,  appear 
to  be  taken  up  by  the  capillary  blood-vessels  of  the  villus, 
so  that  very  little  if  any  of  them  gets  to  the  lacteal  radicle. 
From  the  capillaries  of  the  villi  the  peptones  and  sugar  are 
then  carried  along  the  portal  vein  to  the  liver,  where  they 
probably  undergo  some  further  change.  So  that  while  the 
fat,  though  it  gets  for  the  most  part  into  the  general  blood 
current  by  a  roundabout  way,  viz.,  by  the  lymphatics, 
reaches  the  blood,  as  far  as  we  know,  very  little  changed  ; 
the  peptones  and  sugars  on  the  other  hand,  though  -also 
taking  a  roundabout  course,  viz.,  by  the  liver,  are  probably 
altered    before    they   are    thrown    into    the    general    blood 


vii  SOME   ASPECTS   OF   NUTRITION  291 

stream ;  for  the  portal  blood  in  which  they  are  carried  is 
acted  upon  by  the  liver  before  it  flows  through  the  hepatic 
vein  into  the  general  venous  system.  But  concerning  both 
the  process  of  absorption  itself  and  the  changes  undergone 
by  the  absorbed  products  before  they  reach  the  heart,  ready 
to  be  distributed  all  over  the  body,  we  have  probably  much 
yet  to  learn. 

Part   II.  —  Food  and  Nutrition 

1.  Some  Aspects  of  Nutrition.  —  The  digestive  changes 
which  mixed  food  undergoes  in  the  alimentary  canal  pre- 
pares the  food-stuffs,  of  which  food  is  composed,  for  distri- 
bution to  the  various  tissues  of  the  body.  Entering  the 
tissues,  the  food-stuffs  provide,  by  the  oxidational  changes 
which  they  undergo,  the  energy  which  the  body  expends  as 
heat  and  mechanical  work,  and  at  the  same  time  they  make 
good  the  waste  of  substance  which  results  from  previous 
oxidation.  While  being  in  this  way  metabolised,1  or  worked 
through  the  tissues,  the  food-stuffs  may  give  rise  incidentally 
to  changes  in  the  composition  of  any  individual  tissue  or  of 
a  group  of  tissues,  and  hence  of  the  body  as  a  whole.  These 
latter  changes  depend  partly  upon  the  total  amount  of  mixed 
food  supplied  to  the  body,  but  more  particularly  upon  the 
relative  amounts  of  each  kind  of  food-stuff  which  is  present 
in  and  goes  to  make  up  that  variable  total  amount  of  food. 
Thus,  we  have  the  phenomena  of  starvation  as  an  extreme 
instance  of  the  effect  of  variation  in  the  amount  of  food 
given;  and  the  special  storage  of  glycogen  (p.  243)  and  of 
fat  are  obvious  instances  of  the  effects  of  individual  food- 
stuffs. The  consideration  of  the  possibilities  thus  indicated 
provides  some  of  the  problems  of  nutrition.     But  nutrition 

1  See  note  on  p.  213. 


292  ELEMENTARY   PHYSIOLOGY  less. 

has  also  to  deal  with  the  quantitative  relationships  between 
the  amount  of  food  supplied  and  the  amount  of  waste  ex- 
creted ;  to  strike  a  balance  between  the  two ;  and  to  draw 
conclusions  from  the  balance-sheet  as  to  how  the  business 
of  the  body  is  being  carried  on.  Further,  since  food  not 
only  repairs  waste  but  also  provides  energy,  the  balance- 
sheet  must  take  into  account  how  much  total  energy  is  sup- 
plied in  the  food  and  how  this  available  income  is  expended 
as  heat  and  work. 

2.  Some  Statistics  of  Nutrition.  —  The  average  weight 
of  a  healthy  full-grown  man  may  be  taken  as  70  kilogrammes 
(154  pounds).  Such  a  body  is  made  up,  in  round  numbers, 
as  follows  :  — 

Muscles  and  Tendons 41  per  cent. 

Skeleton 16  " 

Skin 7  " 

Fat 18 

Brain 2  " 

Thoracic  viscera 2  " 

Abdominal  viscera 7  " 

Blood1 7  " 

100        " 

The  waste  of  water  and  other  matter  which  this  body 
excretes  daily  and  their  distribution  among  the  chief  excre- 
tory organs  are  shown  in  the  table  on  the  following  page. 

The  "  other  matter "  from  the  lungs  is  chiefly  carbonic 
acid,  in  which  the  larger  part  of  the  carbon  is  excreted, 
bringing  with  it  nearly  all  the  oxygen  originally  taken  in  by 
the  lungs.  From  the  kidneys  the  "  other  matter  "  includes 
urea,  which    contains    nearly    the    whole    of   the    nitrogen 

1  The  total  amount  of  blood  in  the  body  is  about  5  litres,  or  more  than  a 
gallon,  and  may  be  taken  as  being  usually  about  fa  or  fc  of  the  weight  of  the 
body. 


SOME   STATISTICS   OF  NUTRITION 


293 


> 
Q 
O 
PQ 

< 

s 

D 

W 

w 


o 

o 
o 

E- 
D 
O 

3 

< 
Q 

W 
O 
< 

> 


a 

£ 

U3 
•B.S 

c 
K  "3 

0 

>-     O 

s  S 

zi   O 

s  0 

O 

00 

bo^ 

tuO^ 

ao^ 

N 

0  ^ 

0 

c 


bo  n 


Sis 


294  ELEMENTARY  PHYSIOLOGY  less. 

excreted,  together  with  some  25  grammes  (nearly  1  oz.)  of 
inorganic  salts.  From  the  skin  the  "  other  matter "  is  a 
small  amount  of  salts  and  some  carbonic  acid,  and  in  the 
faeces  it  includes  some  5  grammes  of  salts.  The  total  out- 
put of  salts  from  the  body  is  thus  about  30  grammes  (or 
rather  more  than  1  oz.). 

This  daily  loss  has  to  be  made  good  by  the  new  food 
supplied.  But  in  calculating  the  amount  of  material  neces- 
sary to  replace  the  waste,  we  need  only  turn  our  attention 
to  the  nitrogen  and  the  carbon  ;  for  the  water  lost  represents 
almost  entirely  water  taken  as  drink  or  in  the  food,  the  oxy- 
gen is  that  which  is  derived  from  the  air,  and  the  salts  are 
largely,  though  not  entirely,  introduced  as  salts  with  the  food. 

The  daily  waste  of  nitrogen  and  carbon  may  be  taken  in 
round  numbers  as  about  20  grammes  (300  grains)  of  the 
former  and  270  grammes  (or  about  g\  oz.)  of  the  latter. 
The  nitrogen  necessary  to  make  good  this  loss  can  be 
obtained  only  from  proteids.  The  carbon  may  come  from 
proteids,  carbohydrates,  or  fats,  but  most  advantageously,  as 
we  shall  see,  from  a  mixture  of  all  three. 

The  necessity  of  constantly  renewing  the  supply  of  proteid 
matter  arises  from  the  circumstance  that  the  body  is  unable 
to  employ  for  the  renewal  of  its  proteids  nitrogen  in  any 
other  form  than  proteid  itself.  If  proteid  matter  be  not 
supplied,  the  body  must  needs  waste,  because  then  there  is 
nothing  in  the  food  competent  to  make  good  the  nitro- 
genous loss.  On  the  other  hand,  if  proteid  matter  be  sup- 
plied, there  can  be  no  absolute  necessity  for  any  other  but 
the  mineral  food-stuffs,  because  proteid  matter  contains 
carbon  and  hydrogen  in  abundance,  and  hence  is  competent 
to  make  good  not  only  the  breaking  down  which  is  indicated 
by  the  nitrogenous  loss,  but  also  that  which  is  indicated  by 
the  other  great  products  of  waste,  carbonic  acid  and  water. 


vn  SOME   STATISTICS   OF  NUTRITION  295 

It  has  been  found  advantageous,  however,  to  balance 
the  total  waste  in  some  such  way  as  is  shown  in  the  following 
table  of  daily  income. 

TABLE  SHOWING  THE  AVERAGE  DAILY  INCOME  OF  THE  HUMAN 

BODY 

Nitrogen.  Carbon. 

Proteids        130  grammes  (45  oz.)  contain  20  grammes  (J  oz.)     70  grammes  (25  oz.) 
Fats 
Carbo- 


50 

rammes 

(2  oz.) 

(14  oz.) 

(1  oz.) 

(5  pints) 

- 

40                     (i|oz.) 
160           "         (55  oz.) 

/400 

640 

3,750  g 

20 

grammes 

270  grammes  (9!  oz.) 

Salts 

Water 

Oxygen 

Total 

3.  Diet.  —  Foods,  as  previously  explained  (p.  250),  never 
consist,  except  perhaps  in  the  case  of  fats  and  oils,  of  one 
kind  of  food-stuff  only  ;  each  article  of  food  contains  at  most 
an  excess  of  some  one  kind  of  food-stuff,  and  no  two  foods 
are  exactly  alike.  Hence  the  selection  of  such  foods  as  will 
supply  the  amount  of  proteids,  fats,  and  carbohydrates  re- 
quired by  the  above  statement  opens  up  the  possibility  of  an 
almost  indefinite  choice. 

Suppose  that  we  select  lean  meat,  bread,  potatoes,  milk, 
and  fat,  such  as  butter  or  dripping.  From  the  amounts  of 
each  food  shown  in  the  table  on  the  following  page,  we  may 
obtain  all  that  we  require. 

The  amounts  of  the  several  foods  shown  in  this  table 
suffice  to  cover  the  waste  shown  in  the  table  on  p.  293  and 
constitute  what  is  ordinarily  known  as  a  diet.  But  the  data 
thus  given  are  to  be  taken  rather  as  an  illustration  of  how 
the  balance-sheet  between  food  and  waste  is  drawn  up,  than 
as  an  example  of  exactly  what  a  man  ought  to  eat  in  the 
way  of  food.  As  already  pointed  out,  foods  are  many,  and 
vary  in  the  relative  amounts  of  the  several  food-stuffs  they 
contain,  so  that  it  is  possible  to  draw  up  many  such  tables, 


296 


ELEMENTARY   PHYSIOLOGY 


8 


OJ 


u    in      4-> 


m 

a 

0> 

fl 

0 

~z 

0 

0 

— 

1-4 

"3 

1) 

Pu 

£h 

0 

">*•  ■* 

0 

pa 


7s  ~~ 

?3  >-** 

rt 

s  ° 

|'S. 

So~ 

So 

Sof 

v. 

&^ 

0 

In 

O 

0 

OAg 

8^ 

0  ^ 

0  ^ 

O  >w 

0 

0 

vii  THE   ECONOMY  OF   A   MIXED   DIET  297 

all  satisfying  the  condition  of  covering  the  daily  loss  of  20 
grammes  of  nitrogen  and  about  270  or  300  grammes  of 
carbon. 

In  drawing  up  such  a  table  of  diet  the  question  of  cost 
must  also  not  be  forgotten.  Thus,  for  instance,  it  costs 
more  to  obtain  the  requisite  amount  of  carbon  from  fat 
than  from  sugar  or  starch.  Moreover,  the  value  of  a  diet 
depends  also  on  the  ease  with  which  its  constituents  can  be 
digested  and  utilised.  Mere  chemical  analysis  is  by  itself 
a  very  insufficient  guide  as  to  the  usefulness  and  nutritive 
value  of  an  article  of  food.  A  substance  to  be  nutritious 
must  not  only  contain  some  or  other  of  the  various  food- 
stuffs, but  contain  them  in  an  available,  that  is  a  digestible, 
form.  A  piece  of  beef-steak  is  far  more  nourishing  than  a 
quantity  of  pease  pudding  containing  even  a  larger  propor- 
tion of  proteid  material,  because  the  former  is  far  more 
digestible  than  the  latter;  and  a  small  piece  of  dry  hard 
cheese,  though  of  high  nutritive  value  as  judged  by  mere 
chemical  analysis,  will  not  satisfy  the  more  subtle  criticism 
of  the  stomach. 

4.  The  Economy  of  a  Mixed  Diet.  —  The  body,  as  we 
have  pointed  out,  cannot  obtain  the  nitrogen  it  requires 
from  any  source  other  than  the  ready-made  proteids. 
Hence  in  the  absence  of  these  from  the  food  of  an  animal 
it  must  sooner  or  later  die  from  what  is  known  as  nitrogen 
starvation. 

In  this  case,  and  still  more  in  that  of  an  animal  deprived 
of  food  altogether,  the  organism,  so  long  as  it  continues  to 
live,  feeds  upon  itself.  In  the  former  case,  all  the  processes 
involving  a  loss  of  nitrogen,  in  the  latter,  all  the  processes 
leading  to  the  appearance  of  all  the  several  waste  products, 
are  necessarily  carried  on  at  the  expense  of  its  own  body ; 
whence  it  has  been  rightly  enough  observed  that  a  starving 


298  ELEMENTARY   PHYSIOLOGY  less. 

sheep  is  as  much  a  carnivore  as  a  lion.  Protcid  is  thus  the 
essential  element  of  all  food,  and  since  it  contains  carbon  as 
well  as  nitrogen  it  may  suffice,  under  certain  circumstances, 
to  maintain  the  body ;  but  it  is  a  very  disadvantageous  and 
uneconomical  food-stuff  when  taken  by  itself. 

Albumin,  which  may  be  taken  as  a  type  of  the  proteids, 
contains  about  53  parts  of  carbon  and  15  of  nitrogen  in  100 
parts.  If  a  man  were  to  be  fed  on  white  of  egg,  therefore, 
he  would  take  in,  speaking  roughly,  31  parts  of  carbon  for 
every  part  of  nitrogen. 

But  we  have  seen  that  a  healthy,  full-grown  man,  keeping 
up  his  weight  and  heat,  and  taking  a  fair  amount  of  exer- 
cise, eliminates  per  diem  270  to  300  grammes  of  carbon  to 
only  20  grammes  of  nitrogen,  or,  roughly,  only  needs  one- 
thirteenth  to  one-fifteenth  as  much  nitrogen  as  carbon.  If 
he  is  to  get  his  270  grammes  of  carbon  out  of  albumin,  he 
must  eat  500  grammes  of  that  substance.  But  500  grammes 
of  albumin  contain  75  grammes  of  nitrogen,  or  nearly  four 
times  as  much  as  he  wants. 

To  put  the  case  in  another  way,  it  takes  about  four 
pounds  (1,800  grammes)  of  fatless  meat  (which  generally 
contains  about  one-fourth  its  weight  of  dry  solid  proteids)  to 
yield  the  necessary  amount  of  carbon,  whereas  one  pound 
(453  grammes)  will  furnish  all  the  nitrogen  that  is  required. 

Thus,  a  man  confined  to  a  purely  proteid  diet  must  eat 
an  excessive  quantity  of  it  in  order  to  obtain  the  amount  of 
carbon  he  requires.  This  not  only  involves  a  great  amount 
of  physiological  labour  in  comminuting  the  food,  and  a 
great  expenditure  of  power  and  time  in  dissolving  and 
absorbing  it,  but  throws  a  great  quantity  of  wholly  profitless 
labour  upon  those  excretory  organs  that  have  to  get  rid 
of  the  nitrogenous  matter,  three- fourths  of  which,  as  we  have 
seen,  is  superfluous. 


vii  THE   ECONOMY   OF  A   MIXED   DIET  299 

Unproductive  labour  is  as  much  to  be  avoided  in  physio- 
logical as  in  political  economy ;  and  it  is  quite  possible  that 
an  animal  fed  with  perfectly  nutritious  proteid  matter  should 
die  of  starvation  ;  the  loss  of  power  in  the  various  operations 
required  for  its  assimilation  overbalancing  the  gain  ;  or  the 
time  occupied  in  their  performance  being  too  great  to  per- 
mit waste  to  be  repaired  with  sufficient  rapidity.  The  body, 
under  these  circumstances,  falls  into  the  condition  of  a 
merchant  who  has  abundant  assets,  but  who  cannot  get  in 
his  debts  in  time  to  meet  his  creditors. 

These  considerations  lead  us  to  the  physiological  justifi- 
cation of  the  universal  practice  of  mankind  in  adopting  a 
mixed  diet,  in  which  proteids  are  mixed  either  with  fats  or 
with  carbohydrates,  or  with  both. 

Fats  may  be  taken  to  contain  about  80  per  cent,  of 
carbon,  and  carbohydrates  about  40  per  cent.  Now  it  has 
been  seen  that  there  is  enough  nitrogen  to  supply  the 
waste  of  that  substance  per  diem,  in  a  healthy  man,  in 
453  grammes  (a  pound)  of  fatless  meat,  which  also  contains 
67  grammes  (1,000  grains)  of  carbon,  leaving  a  deficit  of 
200  grammes  (3,000  grains)  of  carbon  ;  250  grammes  (say 
half  a  pound)  of  fat,  or  500  grammes  (rather  more  than  a 
pound)  of  sugar,  will  supply  this  quantity  of  carbon. 

Several  apparently  simple  articles  of  food  constitute  a 
mixed  diet  in  themselves.  Thus,  butcher's  meat  commonly 
contains  from  30  to  50  per  cent,  of  fat.  Bread,  on  the  other 
hand,  contains  the  proteid  gluten,  and  the  carbohydrates, 
starch  and  sugar,  with  minute  quantities  of  fat.  But,  from 
the  proportion  in  which  these  proteid  and  other  constituents 
exist  in  these  substances,  they  are  neither,  taken  alone, 
such  physiologically  economical  foods  as  they  are  when  com- 
bined in  the  proportion  of  about  200  to  75,  or  two  pounds 
of  bread  to  three-quarters  of  a  pound  of  meat  per  diem. 


300  ELEMENTARY   PHYSIOLOG\  less. 

There  is  one  largely  consumed  article  of  food  which  is 
not  merely  composed  of  all  the  various  food-stuffs  requisite 
to  provide  a  mixed  diet,  but  contains  these  substances  in  the 
relative  amounts  very  suitable  for  affording  an  economical 
diet  as  regards  the  proportion  of  the  nitrogen  to  the  carbon. 
This  food  is  milk.  Milk  consists  chiefly  of  water  (86  p.  c.) 
in  which  proteids,  casein,  and  some  albumin  are  dissolved,  as 
also  a  carbohydrate,  milk-sugar  or  lactose,  and  inorganic  salts, 
such  as  chlorides  and  phosphates  of  sodium,  potassium,  and 
calcium.  The  fat  present  in  milk  is  emulsified  or  suspended 
in  the  water  in  the  form  of  extremely  minute  globules,  and 
the  white  appearance  presented  by  milk  is  due  to  the  great 
amount  of  light  reflected  from  these  minute  particles  of  fat. 

5.  The  Effects  of  the  Several  Food-stuffs.  —  When 
proteid  food  is  given  to  an  animal,  such  as  a  dog,  which 
has  been  fasting,  the  larger  part  of  the  nitrogen  given  in 
the  proteid  is  not  retained  in  the  body  but  is  excreted 
almost  immediately.  If  another  larger  meal  of  proteid  be 
given,  the  amount  of  nitrogen  excreted  is  still  further  in- 
creased, less  and  less  being  retained  in  the  body.  By  pro- 
ceeding in  this  way  it  is  possible  to  increase  the  excretion 
of  nitrogen  to  such  an  extent  that  it  ultimately  becomes 
equal  to  the  amount  administered  in  the  food  :  the  animal 
is  then  said  to  be  in  "  nitrogenous  equilibrium."  Such  are 
the  facts,  and  their  meaning  is  obvious.  The  proteid  food 
stirs  up  the  nitrogenous  metabolism  of  the  body  and  stimu- 
lates it  to  an  increased  activity.  But  this  effect  is  not  con- 
fined to  the  nitrogenous  metabolism  alone,  for  if  the  output 
of  carbonic  acid  and  the  corresponding  intake  of  oxygen 
be  measured  during  the  above  experiment,  they  are  both 
found  to  be  considerably  above  the  average.  Hence  pro- 
teid food  also  stimulates  the  non-nitrogenous  metabolism  of 
the  body  and  thus  leads  to  an  increased  waste  of  all  kinds. 


vii     THE   EFFECTS   OF  THE   SEVERAL   FOOD-STUFFS    301 

This  fact  is,  indeed,  made  use  of  for  the  treatment  of  obes- 
ity by  dieting,  as  in  the  Banting  "  cure,"  in  which  the  carbo- 
hydrates and  fat  in  the  food  are  reduced  as  far  as  possible 
and  large  amounts  of  proteid  are  given. 

Because  of  their  lack  of  nitrogen  the  effects  of  carbohy- 
drates and  fats  as  foods  cannot  be  studied  by  feeding  an 
animal  with  these  alone,  as  is  possible  with  proteids.  But 
this  difficulty  may  be  got  over  by  administering  a  small, 
fixed  quantity  of  proteid  with  a  variable  amount  of  either 
carbohydrate  or  fat.  In  this  case  it  is  found  that  an  in- 
crease of  the  carbohydrate  in  food  very  soon  leads  to  the 
laying  on  of  fat ;  and  this  corresponds  to  the  everyday 
experience  which  is  frequently  embodied  in  the  expression 
"  sugar  is  fattening."  At  the  same  time  analysis  of  the  liver 
shows  that  a  large  amount  of  glycogen  is  stored  up  in  it,  as 
previously  explained  (p.  243). 

If  fats  be  given  in  increasing  quantity  they  also  finally 
lead  to  a  laying  on  of  fat,  but  by  no  means  so  readily  as 
does  an  increase  of  carbohydrates.  At  the  same  time,  no 
storage  of  glycogen  is  observed  in  the  liver.  Fats  are  there- 
fore not  as  fattening  as  might  at  first  sight  have  been  ex- 
pected. 

The  salts  which  leave  the  body  are  largely  the  salts  which 
were  introduced  in  the  food.  It  might  therefore  at  first 
sight  appear  that  they  are  merely  unavoidable  constituents 
of  food  which  are  largely  passed  without  change  through  the 
body.  But  this  is  not  the  case.  In  some  way  or  other  the 
salts  of  food  play  an  essential  part  in  directing  the  metab- 
olism taking  place  in  the  tissues.  Thus  animals  fed  with 
an  abundance  of  food,  which  has,  however,  been  freed  as 
far  as  possible  from  salts,  soon  die  with  symptoms  of  de- 
fective nutrition,  accompanied  by  paralysis  and  convulsions. 

When  an  animal  is  deprived  of  all   food  whatsoever,  it 


^02  ELEMENTARY   PHYSIOLOGY  less. 

begins  to  feed  on  its  own  tissues.  Thus  up  to  the  day  of 
its  death  from  starvation  there  is  an  output  of  urea  and 
of  carbonic  acid,  though  in  amounts  less  than  when  food 
is  being  taken.  The  loss  of  tissue-substance  thus  pro- 
duced affects  the  several  tissues  to  different  extents;  but 
without  entering  into  details  we  may  simply  point  out  that 
the  master-tissues  suffer  least,  in  the  obvious  effort  to  pro- 
long life  to  the  utmost.  Thus  the  brain  and  spinal  cord 
are  almost  unaltered  at  death,  and  the  blood  and  the  mus- 
cular tissue  of  the  heart  also  lose  but  little  as  compared 
with  the  fat  and  the  skeletal  muscles. 

6.  The  Erroneous  Division  of  Food-stuffs  into  Heat- 
producers  and  Tissue-formers.  —  Food-stuffs  have  been 
divided  into  heat-producers  and  tissue-formers  —  the  car- 
bohydrates and  fats  constituting  the  former  division,  the 
proteids  the  latter.  But  this  is  a  very  misleading,  and 
indeed  erroneous  classification,  inasmuch  as  it  implies,  on 
the  one  hand,  that  the  oxidation  of  the  proteids  does  not 
develop  heat ;  and,  on  the  other,  that  the  carbohydrates  and 
fats,  in  being  oxidised,  subserve  only  the  production  of  heat. 

Undoubtedly  proteids  are  tissue-formers,  inasmuch  as  no 
tissue  can  be  produced  without  them ;  for  all  the  tissues  are 
nitrogenous,  some  containing  a  large  and  others  a  small 
quantity  of  nitrogen,  and  proteids  are  the  only  nitrogenous 
food-stuffs ;  they  alone  can  supply  the  nitrogenous  elements 
of  the  tissues.  But  there  is  reason  to  think  that  the  fats 
and  carbohydrates  taken  as  food  may  also  be  directly 
built  up  into  the  tissues. 

Moreover,  if  the  proteids  alone  were  the  tissue-formers, 
then  the  energy  set  free  during  the  contracting  activity  of 
the  preeminently  nitrogenous  muscles  ought  to  come  from 
the  metabolism  of  their  proteids.  But  under  most  circum- 
stances this  is  probably  not  the  case,  for  muscular  exercise 


vii     THE   INCOME   AND   EXPENDITURE  OF   ENERGY     303 

does  not  lead  to  any  increased  output  of  nitrogenous  waste 
which  is  in  the  least  proportionate  to  the  work  being  done. 
On  the  other  hand,  exercise  at  once,  and  largely,  increases 
the  excretion  of  carbonic  acid,  to  an  extent  which  may  be 
five  times  as  great  as  during  rest ;  that  is  to  say,  the  non- 
nitrogenous  part  of  the  tissue  seems  to  be  used  up  more 
quickly  than  the  nitrogenous  part ;  and  the  consumption 
of  this  particular  constituent  of  the  muscular  substance  may 
be  made  good  by  non-nitrogenous  food,  by  fats  or  carbo- 
hydrates. 

On  the  other  hand,  proteids  must  be  regarded  as  heat- 
producers  also.  For  though  in  some  tissues,  as  in  muscles, 
the  non-nitrogenous  part  seems  to  be  most  rapidly  changed, 
yet  the  nitrogenous  part,  supplied  by  the  proteids,  is  sooner 
or  later  oxidised,  and  in  being  oxidised  must  give  rise  to 
heat. 

As  soon  as  the  elements  of  the  food,  in  fact,  get  into  the 
tissues,  the  distinction  between  the  two  classes  is  lost ;  both 
form  tissues,  and  both  supply  heat. 

If  it  is  worth  while  to  make  a  special  classification  of  the 
food-stuffs  at  all,  it  appears  desirable  to  distinguish  the  essential 
food-stuffs,  or  proteids,  from  the  accessory  food-stuffs,  or  fats 
and  carbohydrates  —  the  former  alone  being,  in  the  nature 
of  things,  necessary  to  life,  while  the  latter,  however  impor- 
tant, are  not  absolutely  necessary. 

7.  The  Income  and  Expenditure  of  Energy.  —  It  is 
quite  certain  that  nine-tenths  of  the  dry,  solid  food  which 
is  taken  into  the  body  sooner  or  later  leaves  it  in  the  shape 
of  carbonic  acid,  water,  and  urea ;  and  it  is  also  certain  not 
only  that  the  compounds  which  leave  the  body  are  more 
highly  oxidised  than  those  which  enter  it,  but  that  all  the 
oxygen  taken  into  the  blood  by  the  lungs  is  carried  away 
out  of  the  body  in  the  various  waste  products. 


J04  ELEMENTARY   PHYSIOLOGY  less. 

The  intermediate  stages  of  this  conversion  are,  however, 
by  no  means  so  clear.  It  is  highly  probable  that  practically 
all  the  food-stuffs  which  pass  from  the  alimentary  canal  into 
the  blood,  be  they  proteids,  or  fats,  or  carbohydrates,  are 
absorbed  by  some  tissue  or  other  (muscle,  nervous  tissue, 
glandular  tissue,  and  the  like),  before  they  are  oxidised; 
that,  indeed,  it  is  within  some  tissue  or  other  that  they 
suffer  oxidation,  and  that  the  amount  of  oxidation  going  on 
in  the  blood  is  very  small. 

In  the  course  of  its  oxidation,  the  food  not  only  supplies 
the  energy  which  the  body  expends  in  doing  work,  but  also 
the  energy  which,  as  we  have  seen,  the  body  loses  as  heat. 
The  oxidation  of  the  elements  of  the  food  is  indeed  the 
ultimate  source  of  the  heat  of  our  bodies,  all  other  causes 
being  of  little  moment.  About  this  there  can  be  no  doubt, 
and  it  is  a  further  fact  that  the  oxidation  which  thus  gives 
rise  to  heat  is  not  the  oxidation  of  the  elements  of  the  food 
as  they  are  carried  about  in  the  blood,  but  the  oxidation  of 
the  tissues,  more  especially  the  muscles,  into  which  the 
food-stuffs  have  been  built  up,  and  of  which  they  have 
become  an  integral  part.  The  same  may  be  said  regard- 
ing the  source  of  the  energy  expended  in  muscular 
work. 

The  amount  of  mechanical  work  a  man  does  may  be 
determined  with  no  great  difficulty,  whether  we  calculate  it 
as  work  done  in  walking  or  in  turning  some  machine  or  in 
some  other  effort  which  results  in  overcoming  a  resistance. 
This  work  is  measured  in  terms  of  the  resistance  overcome, 
or  weight  lifted,  multiplied  by  the  height  through  which  it  is 
raised.  Thus,  we  speak  of  the  work  done  in  lifting  one 
pound  through  the  height  of  one  foot  as  a  foot-pound  ;  or, 
using  the  metric  system  and  taking  a  kilogramme  (2.2  lbs.) 
and   a   metre   (39.37   inches)  as   the  units  of  weight  and 


yn     THE   INCOME  AND   EXPENDITURE   OF   ENERGY     305 

distance,  we  call  the  unit  of  work  a  kilogramme-metre, 
equal  to  7.23  foot-pounds.  Using  the  latter  unit  we  may 
say  that  a  good  day's  work  is  about  150,000  kilogramme- 
metres. 

The  unit  of  heat  is  the  amount  of  heat  required  to  raise 
the  temperature  of  one  pound  of  water  through  i°  F.  Now, 
as  is  seen  in  all  ordinary  engines,  heat  can  do  work ;  and  it 
is  found  that  one  unit  of  heat  can  do  778  foot-pounds  of 
work.  This  is  called  the  "  mechanical  equivalent  of  heat." 
In  the  metric  system  the  unit  becomes  the  amount  of  heat 
required  to  raise  the  temperature  of  one  gramme  (15.4 
grains)  of  water  through  i°  C.  This  is  called  a  calorie,  and 
the  mechanical  equivalent  of  heat  is  427  gramme-metres. 
Using  these  data  we  can  readily  convert  heat  into  its  equiva- 
lent of  work,  or  vice  versa. 

The  measurement  of  the  amount  of  heat  given  off  by  the 
body  is  by  no  means  easy,  and  the  sources  of  error  are  con- 
siderable. But,  allowing  for  these,  a  rough  determination 
may  be  made ;  the  heat  thus  measured  may  be  calculated 
as  work  by  using  the  mechanical  equivalent  of  heat,  and 
the  result  may  be  added  to  the  actual  work  done  as  work. 
The  outcome  of  this  calculation  shows  that  of  the  total 
energy  expended  by  the  body  about  one-sixth  is  put  out  as 
work  and  five-sixths  as  heat.  Finally,  we  find  that  the  aver- 
age total  output  of  energy  as  work  and  heat  (calculated  as 
work)  may  be  taken  as  about  1,000,000  kilogramme-metres 
daily. 

We  may  now  consider  how  far  this  expenditure  is  met  by 
the  income  of  energy  in  food.  When  a  substance  is  com- 
pletely burnt,  i.e.  oxidised,  to  water  and  carbonic  acid, 
a  certain  amount  of  heat  is  produced,  which  can  be  meas- 
ured. Thus,  it  is  possible  to  determine  how  much  heat  is 
produced  by  the  complete  combustion  of  one  gramme  of 


306  ELEMENTARY   PHYSIOLOGY  less,  vii 

each  of  the  food-stuffs,  proteids,  fats,  and  carbohydrates. 
The  result  obtained  is  as  follows  :  — 

i  gramme  of  proteid  gives  5,700  calories. 

I         "         "  fat  "     9,500       " 

I         "         "  carbohydrate     "     4,000       " 

Now  this  must  also  be  the  amount  of  heat  produced  by  the 
same  quantity  of  each  of  these  food-stuffs  during  their  com- 
plete oxidation  in  the  animal  body.  In  the  case  of  the  pro- 
teid some  deduction  has  to  be  made  because  the  proteids 
are  not  completely  oxidised ;  the  nitrogen  they  contain 
leaves  the  body  as  urea,  which  is  still  capable  of  under- 
going further  oxidation  to  water,  nitrogen  and  carbonic 
acid.  One  gramme  of  proteid  gives  rise  to  about  ^  gramme 
of  urea,  and  the  complete  combustion  of  this  amount  of 
urea  gives  rise  to  844  calories.  Hence,  deducting  these 
from  the  5,700  gives  us  about  4,800  calories,  which  we  may 
take  as  being  the  physiologically  available  heat  of  combus- 
tion of  one  gramme  of  proteid.  If  we  apply  these  values 
to  the  diet  given  on  p.  296  we  find  that :  — 

130  grammes  of  proteid  give     624,000  calories. 

50  "         "  fats  "       475,000       " 

400         "         "  carbohydrate    "     1,600,000       " 

2,699,000 

If  now  we  take  the  mechanical  equivalent  of  this  heat  we 
find  it  works  out  as  1,152,473  kilogramme-metres.  Hence 
the  energy  available  by  the  oxidation  in  the  body  of  this 
particular  diet  is  more  than  sufficient  to  balance  the  total 
amount  which  we  saw  was  expended. 


LESSON   VIII 

MOTION   AND   LOCOMOTION 

1.  The  Source  of  Active  Power  and  the  Organs  of 
Motion.  —  In  the  preceding  Lessons  the  manner  in  which 
the  incomings  of  the  human  body  are  converted  into  its 
outgoings  has  been  explained.  It  has  been  seen  that  new 
matter,  in  the  form  of  organic  and  mineral  food,  is  con- 
stantly appropriated  by  the  body,  to  make  up  for  the  loss 
of  old  matter,  which  is  as  constantly  going  on  in  the  shape, 
chiefly,  of  carbonic  acid,  urea,  and  water,  the  formation 
of  this  waste  being  the  outcome  of  oxidation  accompanied 
by  a  liberation  of  energy. 

The  organic  foods  are  derived  directly,  or  indirectly,  from 
the  vegetable  world  :  and  the  products  of  waste  either  are 
such  compounds  as  abound  in  the  mineral  world,  or  im- 
mediately decompose  into  them.  Consequently,  the  human 
body  is  the  centre  of  a  stream  of  matter  which  sets  inces- 
santly from  the  vegetable  and  mineral  worlds  into  the  min- 
eral world  again.  It  may  be  compared  to  an  eddy  in  a 
river,  which  may  retain  its  shape  for  an  indefinite  length 
of  time,  though  no  one  particle  of  the  water  of  the  stream 
remains  in  it  for  more  than  a  brief  period. 

But  there  is  this  peculiarity  about  the  human  eddy,  that 
a  large  portion  of  the  particles  of  matter  which  flow  into 
it  have  a  much  more  complex  composition  than  the  parti- 
cles which  flow  out  of  it.     To  speak  in  what  is  not  altogether 

3°7 


3o8 


ELEMENTARY   PHYSIOLOGY 


LESS. 


a  metaphor,  the  atoms  enter  the  body,  for  the  most  part, 
piled  up  into  large  heaps,  and  tumble  down  into  small  heaps 
before  they  leave  it.  The  energy  which  they  set  free  in 
this  tumbling  down,  is  the  source  of  the  active  powers  of 
the  organism. 

These  active  powers  are  chiefly  manifested  in  the  form 
of  motion  —  movement,  that  is,  either  of  part  of  the  body, 
or  of  the  body  as  a  whole,  which  last  is  termed  locomotion. 

The  organs  which  produce  total  or  partial  movements 
of  the    human   body  are    of  three    kinds  :    cells  exhibiting 

amoeboid  movejnents,    cilia,    and 
muscles. 

The  amoeboid  movements  of 
the  white  corpuscles  of  the  blood 
have  been  already  described  (p. 
126),  and  it  is  probable  that 
similar  movements  are  performed 
by  many  other  simple  cells  of  the 
body  in  various  regions. 

The  amount  of  movement  to 
which  each  cell  is  thus  capable 
of  giving  rise  may  appear  per- 
fectly insignificant ;  nevertheless,  there  are  reasons  for  think- 
ing that  these  amoeboid  movements  are  of  great  importance 
to  the  economy,  and  may  under  certain  circumstances  be 
followed  by  very  notable  consequences. 

2.  Ciliated  Epithelium  and  the  Action  of  Cilia.  —  Cilia 
are  filaments  of  extremely  small  size,  attached  by  their 
bases  to,  and  indeed  growing  out  from,  the  free  surfaces 
of  certain  epithelial  cells ;  there  being  in  most  instances 
very  many  (thirty  for  instance),  but,  in  some  cases,  only 
a  few  cilia  on  each  cell  (Figs.  49,  90).  In  some  of  the 
lower  animals,  cells  may  be  found  possessing  only  a  single 


Fig.  90.  —  Columnar  Ciliated 
Epithelium  Cells  from  the 
Human  Nasal  Membrane. 

Magnified  300  diameters. 
(Sharpey.) 


vin     CILIATED   EPITHELIUM   AND  ACTION  OF  CILIA     309 

cilium.  Cilia  are  in  incessant  waving  motion,  so  long  as 
life  persists  in  them.  Their  most  common  form  of  move- 
ment is  that  "in  which  each  cilium  is  suddenly  bent  upon 
itself,  becomes  sickle-shaped  instead  of  straight,  and  then 
more  slowly  straightens  again,  both  movements,  however, 
being  extremely  rapid  and  repeated  about  ten  times  or  more 
every  second.  These  two  movements  are  of  course  an- 
tagonistic ;  the  bending  drives  the  water  or  fluid  in  which 
the  cilium  is  placed  in  one  direction,  while  the  straightening 
drives  it  back  again.  Inasmuch,  however,  as  the  bending 
is  much  more  rapid  than  the  straightening,  the  force  ex- 
pended on  the  water  in  the  former  movement  is  greater 
than  in  the  latter.  The  total  effect  of  the  double  move- 
ment, therefore,  is  to  drive  the  fluid  in  the  direction  towards 
which  the  cilium  is  bent :  that  is,  of  course,  if  the  cell  on 
which  the  cilia  are  placed  is  fixed.  If  the  cell  be  floating 
free,  the  effect  is  to  drive  or  row  the  cell  backwards ;  for 
the  cilia  may  continue  their  movements  even  for  some 
time  after  the  epithelial  cell,  with  which  they  are  connected, 
is  detached  from  the  body.  And  not  only  do  the  move- 
ments of  the  cilia  thus  go  on  independently  of  the  rest  of 
the  body,  but  they  appear  not  to  be  controlled  by  the 
action  of  the  nervous  system.  Each  cilium  is  comparable 
to  one  of  the  mobile  processes  of  a  white  corpuscle.  A 
ciliated  cell  differs  from  an  amoeboid  cell  in  that  its  con- 
tractile processes  are  permanent,  have  a  definite  shape,  and 
are  localised  in  a  particular  part  of  the  cell,  and  that  the 
movements  of  the  processes  are  performed  rhythmically 
and  always  in  the  same  way.  But  the  exact  manner  in 
which  the  movement  of  a  cilium  is  brought  about  is  not 
as  yet  thoroughly  understood. 

Although  no  other  part  of  the  body  has  any  control  over 
the  cilia,  and  though,  so   far  as  we  know,  they  have  no 


310  ELEMENTARY   PHYSIOLOGY,  less. 

direct  communication  with  one  another,  yet  their  action 
is  directed  towards  a  common  end  —  the  cilia,  which  cover 
extensive  surfaces,  all  working  in  such  a  manner  as  to  sweep 
whatever  lies  upon  that  surface  in  one  and  the  same  direc- 
tion. Thus,  the  cilia  which  are  developed  upon  the  epithelial 
cells  which  line  the  greater  part  of  the  nasal  cavities  and 
the  trachea,  with  its  ramifications,  tend  to  drive  the  mucus 
in  which  they  work,  outwards. 

In  addition  to  the  air-passages,  cilia  are  found,  in  the 
human  body,  in  a  few  other  localities ;  but  the  part  which 
they  play  in  man  is  insignificant  in  comparison  with  their 
function  in  the  lower  animals,  among  many  of  which  they 
become  the  chief  organs  of  locomotion. 

3.  The  Structure  of  Unstriated  Muscle.  —  It  is  custom- 
ary to  distinguish  three  varieties  of  muscle,  unstriated,  cardiac, 
and  striated,  which  differ  from  one  another  in  structure  and 
in  some  respects  in  mode  of  action.  Cardiac  muscle,  which 
occurs  in  the  heart  only,  has  been  already  described  (p.  74). 


Fig.  91.  —  A  Fibre-cell  from  the  Plain,  Non-striated  Muscular  Coat  of 

the  Intestine. 

f,  fibre;   u,  nucleus;  /,  granular  protoplasm  around  the  nucleus. 

Unstriated  (also  called  "  plain "  or  "  smooth  ")  muscle 
occurs  in  the  walls  of  the  alimentary  canal,  the  blood-vessels, 
the  bladder,  and  other  organs.  It  is  composed  of  bundles 
of  fibres,  which  are  bound  together  by  connective  tissue 
carrying  nerves  and  blood-vessels.  The  fibres  are  in  reality 
elongated,  spindle-shaped  cells  whose  length  is  about  100/x 
(yto  mcn)  an(l  width  6/x  (-40V0-  inch).  Somewhere  towards 
the  middle  of  each  cell  there  is  an  elongated  oval  or  some- 


vin  THE   STRUCTURE   OF   STRIATED    MUSCLE  311 

times  rod-shaped  nucleus,  surrounded  by  a  small  amount 
of  granular  protoplasm. 

The  substance  of  the  cell  is  clear  and  shows  no  transverse 
striations,  although  it  often  shows  signs  of  a  very  fine  longi- 
tudinal fibrillation.  Each  cell  is  said  to  be  surrounded  by 
an  extremely  delicate  sheath,  but,  as  to  this,  opinions  differ. 
A  number  of  such  fibre-cells  are  united  together  by  a  mi- 
nute quantity  of  cement,  or  intercellular  substance,  into  a 
thin  fiat  band,  and  a  number  of  such  bands  are  bound 
together  by  connective  tissue  into  larger  bands  or  bundles. 
Each  fibre  is  capable  of  contracting,  that  is,  of  shortening 
and  becoming  at  the  same  time  thicker. 

4.  The  Structure  of  Striated  Muscle.  —  Striated  muscle 
is  also  made  up  of  fibres,  though  the  fibres  are  very  differ- 
ent from  the  fibres  or  fibre-cells  of  unstriated  muscle,  and 
these  fibres  are  again  similarly  bound  up  together  in  various 
ways  by  connective  tissue,  which  carries  the  blood-vessels  and 
nerves,  so  as  to  form  muscles  of  various  shapes  and  sizes.1 
Each  muscle  is  thus  made  up  of  (i)  an  external  wrapping 
or  perimysium;  this  is  a  sheath  of  connective  tissue  from 
the  inner  face  of  which  partitions  proceed  and  divide  the 
space  which  it  incloses  into  a  great  number  of  longitudi- 
nally disposed  compartments  ;  (ii)  the  muscular  fibres  which 
occupy  these  compartments  ;  (iii)  the  vessels  which  lie  in 
the  sheath  and  in  the  partitions  between  the  compartments, 
and  thus  surround  the  muscular  fibres  without  entering  them  ; 
(iv)  the  nerves  which  also  at  first  lie  in  the  sheath  and  in 
the  partitions  between  the  compartments,  but  which  eventu- 
ally enter  into  the  muscular  fibres. 

1  It  is  necessary  to  distinguish  "muscle"  as  an  organ  from  "muscle" 
as  a  tissue.  The  biceps  muscle  (p.  326),  for  example,  is  an  organ  of  a 
complicated  character,  of  which  muscular  tissue  forms  only  the  chief  con- 
stituent. 


312  ELEMENTARY   PHYSIOLOGY  less 

The  perimysium  forms  a  complete  envelope  around  the 
muscle,  which,  when  it  is  sufficiently  strong  to  be  dissected 
off,  is  known  as  a  fascia ;  at  each  end  it  usually  terminates 
in  dense  connective  tissue  (tendon),  which  becomes  con- 
tinuous with  the  bone  or  cartilage  to  which  the  tendon  is 
attached.  The  partitions  given  off  from  the  inner  surface 
of  the  perimysium  form  at  first  coarse  compartments,  inclos- 
ing large  bundles  of  fasciculi  (Fig.  92),  each  consisting  of 
a  very  great  number  of  fibres.  These  large  bundles  are 
again  divided  by  somewhat  finer  connective-tissue  parti- 
tions into  smaller  bundles,  and  these  again  into  still  smaller 


Fig.  92.  —  Fasciculi  of  Striated  Muscle  cut  across. 
Several  fasciculi,  f,  bound  together  into  large  fasciculi  to  make  up  the  muscle. 

ones,  and  so  on,  the  smallest  bundles  of  all  being  composed 
of  a  number  of  individual  muscular  fibres.  In  this  way  the 
partitions  become  thinner  and  more  delicate,  until  those 
which  separate  the  chambers  in  which  the  individual  mus- 
cular fibres  are  contained  are  reduced  to  little  more  than 
as  much  connective  tissue  as  will  hold  the  small  nerves, 
arteries,  veins,  and  capillary  network  together.  As  the 
perimysium  consists  of  connective  tissue,  it  may  be  destroyed 
by  prolonged  boiling  in  water.  In  fact,  in  "  meat  boiled  to 
rags  "  we  have  muscles  which  have  been  thus  treated  :  the 


THE   STRUCTURE   OF   STRIATED    MUSCLE 


3*3 


perimysial  case  is  broken  up,  and  the  muscular  fibres,  but 
little  attacked  by  boiling  water,  are  readily  separated  from 
one  another. 

If  a  piece  of  muscle  of  a  rabbit  which  has  been  thus 
boiled  for  many  hours  is  placed  in  a  watch-glass  with  a 
little  water,  the  muscular  fibres  may  be  easily  teased  out 
with  needles  and  isolated.  Such  a  fibre  will  be  found  to 
have  a  thickness  of  somewhere  about  6o/a  (4-^0  inch)  (they 


Fig.  93.  —  Capillaries  of  Striated  Muscle. 

A.  Seen  longitudinally.  The  width  of  the  meshes  corresponds  to  that  of  a  muscle 
fibre,     a,  small  artery;  i,  small  vein. 

B.  Transverse  section  of  striated  muscle,  a,  the  cut  ends  of  the  fibres;  5,  capil- 
laries filled  with  injection  material;  c,  parts  where  the  capillaries  are  absent  or  not 
filled. 


vary,  however,  a  great  deal),  with  a  length  of  30  or  40  milli- 
metres, i.e.  about  1^-  inch.  It  is  a  cylindroidal  or  polygonal 
solid  rod,  which  either  tapers  or  is  bevelled  off  at  each  end. 
By  these  it  adheres  to  those  on  each  side  of  it ;  or,  if  it  lies 
at  the  end  of  a  series,  to  the  tendon. 

The  structure  and  properties  of  striated  muscular  tissue 


314 


ELEMENTARY   PHYSIOLOGY 


in  the  histological  sense  mean  the  structure  and  properties 
of  these  fibres. 

As  we  have  already  had  occasion  to  remark,  all  tissues 
undergo  considerable  alteration  in  passing  from  the  living  to 
the  dead  state,  but,  in  the  case  of  muscle,  the  changes  which 
the  tissue  undergoes  in  dying  are  of  such  a  marked  charac- 
ter that  the  structure  of  the  dead  tissue  gives  a  false  notion 
of  that  of  the  living  tissue. 


«- 


B 


i  am       4£V«5£E 

'•J    .  ■  ',"":■  , 

;,—  ■-■■         U     (,■.  ,;     • 


Fig.  94.  —  To  illustrate  the  Structure  of  a  Striated  Muscle  Fibre. 

A.  Part  of  a  muscle  fibre  (of  a  frog)  seen  in  a  natural  condition,  d,  dim  bands; 
b,  bright  bands,  with  the  granular  line  seen  in  many  of  them ;  «,  nuclei  and  the  granu- 
lar protoplasm  belonging  to  them,  very  dimly  seen. 

B.  Portion  of  prepared  mammalian  muscle  fibre  teased  out,  showing  longitudi- 
nal portions  of  variable  (1,  2,  3,  4)  thickness;  4  represents  the  finest  portion  (fibrilla) 
which  could  be  obtained;  d,  dim  bancis;  b,  bright  bands,  in  the  midst  of  each  of 
which  is  seen  the  granular  line  g. 

A  living  striated  muscle  fibre  of  a  frog  or  a  mammal  is  a 
pale  transparent  rod  composed  of  a  soft,  flexible,  elastic  sub- 
stance, the  lateral  contours  of  which,  when  the  fibre  is  viewed 
out  of  the  body,  appear  sharply  defined,  like  those  of  a  glass 
rod  of  the  same  size  ;  but  when  the  fibre  is  observed  in  the  liv- 
ing body,  bathed  in  the  lymph  which  surrounds  it,  the  out- 


vin  THE   STRUCTURE   OF   STRIATED    MUSCLE  315 

lines  are  not  so  sharply  defined.  In  neither  case  can  any 
distinct  line  of  demarcation  between  a  superficial  layer  and 
a  deeper  substance  be  recognised.  The  fibre  appears  trans- 
versely striped,  as  if  the  clear  glassy  substance  were,  at 
regular  intervals  (Fig.  94,  A,  d),  converted  into  ground 
glass,  thus  appearing  dimmer.  Each  of  these  "  dim  bands  " 
is  about  2fx  wide,  and  the  clear  space  or  "  bright  band  " 
which  separates  every  two  dim  bands  is  of  about  the  same 
size,  or  under  ordinary  circumstances  somewhat  narrower. 
With  a  high  power  a  very  thin  dark  granular  line  equidistant 
from  each  dim  band  is  discernible  in  each  bright  band, 
dividing  the  bright  band  into  two.  As  these  appearances 
remain  when  the  objective  is  focussed  through  the  whole 
thickness  of  the  fibre,  it  follows  that  the  dim  bands,  the 
granular  lines,  and  the  clear  spaces  on  each  side  of  each 
granular  line,  represent  the  edges  of  segments  of  different 
optical  characters,  which  regularly  alternate  through  the 
whole  length  of  the  fibre.  Let  the  excessively  thin  seg- 
ments, of  which  the  thin  granular  lines  represent  the  edges, 
be  called  g,  the  thicker,  pellucid  segments,  of  which  the 
bright  bands  on  each  side  of  a  granular  line  represent  the 
edges,  b ;  and  the  thickest,  slightly  opaque  segments,  of 
which  the  ground-glass-like  dim  bands  are  the  edges,  d. 
Then  the  structure  of  the  fibre  may  be  represented  by  d.  b. 
g.  b.  d.  b.  g.  b.,  indefinitely  repeated,  and  one  inch  of  length 
of  fibre  will  contain  about  30,000  such  segments,  or  alterna- 
tions of  structure. 

In  a  perfectly  unaltered  living  fibre  the  striated  substance 
presents  hardly  any  sign  of  longitudinal  striation ;  but  near 
to  the  surface  of  the  fibre  in  mammalian  muscle,  though  at 
various  points  in  the  depth  of  the  fibre  in  the  muscles  of 
the  frog,  faint  indications  are  to  be  observed  of  the  existence 
of  nuclej,  each  surrounded  by  a  small  amount  of  granular 
protoplasm  (Fig.  94,  A,  ;/). 


316 


ELEMENTARY   PHYSIOLOGY 


If  the  muscle  fibre  be  preserved  and  studied  by  the  ac- 
cepted histological  methods,  the  nuclei  can  be  made  much 
more  conspicuous ;  and,  moreover,  parallel,  longitudinal 
striae   appear   in   greater  or  less  numbers,  until  sometimes 

the  striated  substance  seems  broken  up  into  fine   delicate 

m 
fibrils,  each  of  which  presents  the  same  segmentation  as  the 

whole  fibre  (Fig.  94,  B,  4).  Transverse  sections  of  mus- 
cular fibre  present  minute  close-set  circu- 
lar dots,  which  appear  to  represent  the 
transverse  sections  of  naturally  existing 
longitudinal  fibrils.  The  most  reasonable 
interpretation  of  these  facts  is  that  the 
fibre  is  really  made  up  of  fibrils,  and  that 
these  are  invisible  in  the  living  muscle  on 
account  of  their  having  the.  same  refrac- 
tive power  as  the  interfibrillar  substance. 
By  proper  treatment  of  the  fibre  there 
may  be  demonstrated  a  thin  membrane 
of  glassy  transparency,  the  sarcolemma, 
which  ensheathes  the  striated  and  fibril- 
lated  substance  (Fig.  95,  s). 

These  are  the  most  important  struc- 
tural appearances  presented  by  ordinary 
striated  muscle.  But  it  may  be  noticed 
further  that  the  dim  bands  exert  a  power- 
ful influence  on  polarised  light.  Hence  when  a  piece  of 
muscle  is  placed  in  the  field  of  a  polarising  microscope  and 
the  prisms  are  crossed  so  that  the  field  is  dark,  these  bands 
appear  bright.  The  granular  lines  have  a  similar  but  very 
much  less  marked  effect. 

In  the  embryo  the  place  of  the  adult  tissue  is  occupied 
by  a  mass  of  closely  applied,  undifferentiated,  nucleated 
cells.     As  development  proceeds,  some  of  these  cells  are 


Fig.  95. —A  Mus- 
cular Fibre  (of 
Frog)  ending  in 
Tendon. 

The  striated  mus- 
cular substance,  m, 
has  shrunk  from  the 
sarcolemma,  s,  the 
fibrils  of  the  tendon, 
/,  being  attached  to 
the  latter. 


vin  THE   STRUCTURE   OF    STRIATED    MUSCLE  317 

converted  into  the  tissues  of  the  perimysium,  but  others, 
increasing  largely  in  size,  gradually  elongate  and  take  on  the 
form  of  more  or  less  spindle-shaped  rods  or  fibres.  Mean- 
while the  nucleus  of  each  cell  repeatedly  divides,  and  thus 
each  rod  becomes  provided  with  many  nuclei,  so  that  each 
fibre  is  really  a  multi-nucleate  cell.  Along  with  these 
changes  the  protoplasmic  substance  of  the  original  cell 
becomes,  for  the  most  part,  converted  into  the  character- 
istically striated  muscle  substance,  only  a  little  remaining 
unaltered  around  each  nucleus. 

The  many-nucleated  cell  thus  changed  into  a  muscle 
fibre  is  nourished  by  the  fluid  exuded  from  the  adjacent 
capillaries,  and  it  may  be  said  to  respire,  inasmuch  as  its 
substance  undergoes  slow  oxidation  at  the  expense  of  the 
oxygen  contained  in  that  fluid,  and  gives  off  carbonic  acid. 
It  is,  in  fact,  like  the  other  elements  of  the  tissues,  an 
organism  of  a  peculiar  kind,  having  its  life  in  itself,  but 
dependent  for  the  permanent  maintenance  of  that  life  upon 
the  condition  of  being  associated  with  other  such  elementary 
organisms,  through  the  intermediation  of  which  its  tempera- 
ture and  its  supply  of  nourishment  are  maintained. 

The  special  property  of  a  living  muscle  fibre,  that  which 
gives  it  its  physiological  importance,  is  its  peculiar  contrac- 
tility. The  body  of  a  colourless  blood  corpuscle,  as  we 
have  seen,  is  eminently  contractile,  inasmuch  as  it  under- 
goes incessant  changes  of  form.  But  these  changes  take 
place  at  all  points  of  its  surface,  and  have  no  definite  rela- 
tion to  the  diameter  of  the  corpuscle,  while  the  contractility 
of  the  muscular  fibre  is  manifested  by  a  diminution  in  the 
length  and  a  corresponding  increase  in  the  thickness  of  the 
fibre.  Moreover,  under  ordinary  circumstances,  the  change 
of  form  is  effected  very  rapidly,  and  only  in  consequence  of 
the  application  of  a  stimulus. 


318  ELEMENTARY   PHYSIOLOGY  less. 

When  a  contracting  striated  fibre  is  observed  under  the 
microscope,  all  the  bands  become  broader  (across  the 
fibre)  and  shorter  (along  the  fibre)  and  thus  more  closely 
approximated.  Some  observers  think  that  the  clear  bands 
are  diminished  in  total  bulk  relatively  to  the  dim  bands  ; 
but  this  is  disputed  by  others.  When  the  fibre  relaxes 
again  the  bands  return  to  their  previous  condition. 

5.  The  Chemistry  of  Muscle.  —  If  a  muscle  taken  per- 
fectly fresh  from  the  body  be  cooled  down  with  ice  in  order 
to  keep  it  from  undergoing  change  (just  as  was  previously 
done  with  blood,  p.  134)  and  subjected  to  considerable 
pressure  it  yields  a  fluid  called  muscle-plasma.  This  re- 
mains fluid  so  long  as  it  is  kept  adequately  cooled,  but  clots 
spontaneously  at  ordinary  temperatures.  The  clotting  takes 
place  in  a  way  very  similar  to  that  already  described  for 
blood-plasma,  and  results  in  the  formation  of  a  semi-solid 
gelatinous  substance,  called  myosin,  and  a  small  amount  of 
fluid,  or  muscle-serum.  Myosin  is  a  proteid  and  belongs  to 
the  same  class  of  proteids  as  do  the  fibrinogen  and  serum- 
globulin  of  blood,  namely  the  globulins.  During  the  for- 
mation of  myosin,  the  fluid,  which  when  first  squeezed  out 
was  faintly  alkaline,  becomes  distinctly  acid  owing  to  the 
formation  of  an  organic  acid  called  sarcolactic  acid.  At  a 
longer  or  shorter  time  after  death  this  clotting  takes  place 
in  the  body  within  the  muscles  themselves.  They  become 
more  or  less  opaque,  and,  losing  their  previous  elasticity, 
set  into  hard,  rigid  masses,  which  retain  the  form  which 
they  possess  when  the  clotting  commences.  Hence  the 
limbs  become  fixed  in  the  position  in  which  death  found  them, 
and  the  body  passes  into  the  condition  of  what  is  termed 
the  '-death-stiffening,"  or  rigor  mortis.  This  stiffening  is 
also  accompanied  by  a  change  in  the  chemical  reaction  of 
the  muscle,  for,  while  living  muscle  when  tested  with  litmus 


vin  THE  CHEMISTRY   OF   MUSCLE  319 

is  faintly  alkaline  or  neutral,  at  least  when  at  rest,  it  becomes 
distinctly  acid  as  rigor  mortis  sets  in.  And  it  may  be  added 
that  a  similar  but  slighter  acidity  is  developed  even  in  a 
living  muscle,  when  it  contracts. 

After  the  lapse  of  a  certain  time  the  coagulated  matter 
liquefies,  and  the  muscles  pass  into  a  loose  and  flabby 
condition,  which  marks  the  commencement  of  putrefaction. 

It  has  been  observed  that  the  sooner  rigor  mortis  sets 
in,  the  sooner  it  is  over ;  and  the  later  it  commences,  the 
longer  it  lasts.  The  greater  the  amount  of  muscular  exer- 
tion and  consequent  exhaustion  before  death,  the  sooner 
rigor  mortis  sets  in. 

Rigor  mortis  evidently  presents  some  analogies  with  the 
clotting  of  the  blood.  Moreover,  the  substance  which  is 
formed  within  the  fibre  (myosin)  is  in  many  respects  not 
unlike  fibrin,  and  is  thought  to  come  from  a  substance  called 
myosinogen,  which  is  believed  to  exist  in  the  living  muscle. 

Besides  myosin,  muscle  contains  other  varieties  of  pro- 
teid  material,  about  which  we  at  present  know  little ;  a 
variable  quantity  of  fat ;  certain  inorganic  saline  matters, 
phosphates  and  potash  being,  as  is  the  case  in  the  red 
blood-corpuscles,  in  excess ;  and  a  large  number  of  sub- 
stances existing  in  small  quantities,  and  often  classed 
together  as  "  extractives."  Some  of  these  extractives  con- 
tain nitrogen  ;  the  most  important  of  this  class  is  creatin, 
a  crystalline  body  which  is  supposed  to  be  the  chief  form  in 
which  nitrogenous  waste  matter  leaves  the  muscle  on  its  way 
to  become  urea. 

The  other  class  of  extractives  contains  bodies  free  from 
nitrogen,  perhaps  the  most  important  of  which  are  sarcolac- 
tic  acid  and  glycogen. 

Most  muscles  are  of  a  deep,  red  colour  ;  this  is  due  in 
part  to  the  blood  remaining  in  their  vessels ;  but  only  in 


320  ELEMENTARY   PHYSIOLOGY  less 

part,  for  each  fibre' (into  which  no  capillary  enters)  has 
a  reddish  colour  of  its  own,  like  a  blood-corpuscle,  but 
fainter.  And  this  colour  is  due  to  the  fibre  possessing  a 
small  quantity  of  that  same  haemoglobin  in  which  the  blood- 
corpuscles  are  so  rich. 

6.  The  Phenomena  of  Muscular  Contraction.  —  Every 
fibre  in  a  muscle  has  the  property,  under  certain  conditions, 
of  shortening  in  length,  while  it  increases  correspondingly 
in  width,  so  that  the  volume  of  the  fibre  remains  unchanged. 
This  property  is  called  muscular  contractility,  and  when- 
ever, in  virtue  of  this  property,  a  muscle  fibre  contracts  it 
tends  to  bring  its  two  ends  closer  together.  Since  a  muscle 
is  made  up  of  a  collection  of  these  fibres,  when  the  fibres 
contract  the  muscle  as  a  whole  also  contracts ;  it  becomes 
shorter  and  thicker,  and  brings  its  two  ends  closer  together, 
along  with  whatever  may  be  fastened  to  those  ends.  By 
this  action  the  muscles  lead  to  the  motion  of  the  parts  to 
which  they  are  attached  and  by  these  motions  give  rise  to 
locomotion  or  other  activities. 

The  condition  which  ordinarily  determines  the  contraction 
of  a  muscle  fibre  is  the  passage  along  the  nerve  fibre,  which 
is  in  close  anatomical  connection  with  the  muscle  fibre,  of  a 
nervous  impulse,  i.e.  of  a  particular  change  in  the  substance 
of  the  nerve,  which  is  propagated  from  particle  to  particle 
along  the  fibre.  The  nerve  fibre  is  called  a  motor  fibre, 
because  by  its  influence  on  a  muscle  it  becomes  the  indirect 
means  of  producing  motion  (see  Lesson  XII.). 

The  phenomena  of  muscular  contraction  may  be  con- 
veniently studied  in  the  large  muscle  from  the  calf  of  a  frog's 
leg,  which,  since  the  frog  is  a  "  cold-blooded "  animal, 
retains  its  power  of  contracting  for  some  time  after  it  is 
removed  from  the  body.  This  muscle  is  called  the 
gastrocnemius,    and    may   be    dissected   out   so   as   to   be 


nil         PHENOMENA   OF   MUSCULAR  CONTRACTION        321 

still  attached  to  a  piece  of  the  femur  near  the  knee,  and  to 
the  nerve,  the  sciatic,  which  supplies  it.  The  preparation  as 
thus  taken  out  of  the  body  is  known  as  a  muscle-nerve  prep- 
aration (Fig.  96). 

The  muscle  may  be  suspended  by  the  femur,  and  a  weight 
be  hung  on  the  tendon  at  its  lower  end,  and  then  the  muscle 


Fig.  96.  —  A  Muscle-nerve  Preparation. 

m,  the  muscle,  gastrocnemius  of  frog;  Sp.c,  lower  end  of  spinal  column;  n,  the 
sciatic  nerve,  all  the  branches  being  cut  away  excepting  that  supplying  the  muscle; 
f,  the  femur;  cl.  a  clamp  to  hold  the  femur;   t.a.  tendon  of  Achilles. 

may  be  made  to  contract  by  stimulating  the  sciatic  nerve 
(see  also  Lesson  XII.). 

When  the  nerve  is  excited  by  a  very  brief  stimulus,  as,  for 
instance,  by  the  momentary  electric  current  often  called  an 
X 


322  ELEMENTARY    PHYSIOLOGY  less. 

induction  shock,  the  following  changes   take  place   in  the 
muscle  :  — 

(i)  It  becomes  shorter  and  thicker,  lifts  the  weight 
attached  to  it  and  then  relaxes,  allowing  the  weight  to  fall 
again.  The  shortening  and  relaxing  take  place  very  rapidly, 
the  whole  process  occupying  rather  more  than  j1^  of  a  second. 

(ii)  The  muscle  may  be  inclosed  in  a  small  chamber 
and  made  to  contract  several  times.  If  now  we  examine 
the  air  in  the  chamber  in  which  this  excised  muscle  has  been 
contracting,  we  cannot  obtain  satisfactory  evidence  of  any 
escape  of  carbonic  acid  from  the  muscle  during  its  con- 
traction; if  carbonic  acid  is  produced  it  must  be  retained 
within  the  muscle,  presumably  in  the  form  of  some  simple 
chemical  compound.  That,  however,  the  muscle  within  the 
body  does  give  off  carbonic  acid  in  some  form  during  its 
contraction  is  wholly  probable.  The  substance  of  the 
muscle  may  at  the  same  time  have  become  faintly  acid,  as 
tested  by  litmus  paper.  The  acidity  is  due  to  sarcolactic 
acid. 

(iii)  The  muscle  becomes  slightly  warmer ;  this  can 
only  be  due  to  the  fact  that  heat  is  formed  during  the 
contraction. 

(iv)  The  muscle  undergoes  certain  electrical  changes. 
At  the  moment  of  commencing  contraction  the  muscle 
becomes  like  a  small  battery  cell,  and  generates  a  current 
of  electricity,  which  can  be  readily  detected.1 

We  have  already  more  than  once  insisted  on  the  fact 
that  all  the  tissues  of  the  body  are  continually  taking  up 
oxygen  which  they  stow  away  in  the  form  of  some  com-, 
pound,  since  no  oxygen  can  be  extracted   from    them  by 

l  See  also  p.  502,  where  the  electrical  changes  of  an  active  nerve,  which 
are  essentially  the  same  as  those  of  a  contracting  muscle,  are  described 
in  greater  detail. 


nil         THE  TETANIC   CONTRACTION   OF   MUSCLES         32:, 

an  air  pump.  Tn  muscle  this  storage  of  oxygen  leads  to 
an  instability  of  the  contractile  substance  of  which  it  is 
composed,  so  that  when  the  appropriate  stimulus  is  given 
to  it,  this  unstable  substance  undergoes  a  sudden  decom- 
position, almost  explosive  in  its  nature ;  and  the  energy  set 
free  during  the  decomposition  makes  itself  known  partly  as 
the  work  which  the  muscle  can  do  in  overcoming  a  resist- 
ance and  partly  as  heat.  This  decomposition  is  accom- 
panied by  an  electrical  disturbance  and  the  appearance  of 
the  products  of  decomposition. 

7.  The  Tetanic  Contraction  of  Muscles.  —  When  experi- 
menting with  a  muscle-nerve  preparation,  as  in  the  preced- 
ing section,  it  is  easy  to  stimulate  the  nerve  twice  in  such 
rapid  succession  that  the  second  stimulus  is  given  while  the 
muscle  is  in  a  state  of  contraction  resulting  from  the  first. 
In  this  case  the  muscle  responds  to  the  second  stimulus  as 
well  as  to  the  first ;  in  other  words,  while  already  contract- 
ing, it  contracts  still  more.  The  second  contraction  is  rather 
less  in  amount  than  the  first,  and  is  added  to  the  first.  If 
a  rapidly  successive  series  of  stimuli  be  applied  to  the  nerve, 
the  muscle  responds  by  an  equally  rapid  series  of  contrac- 
tions, each  of  which  takes  place  before  the  preceding  one 
is  over ;  the  contractions  are  thus  added  together,  and  the 
muscle  remains  in  a  state  of  continued  contraction  as  long 
as  the  stimuli  are  continued,  until  exhaustion  sets  in.  A 
prolonged  contraction  made  up  of  such  a  series  of  single 
contractions  superadded  to  one  another  is  called  a  tetanic 
contraction.  The  acidity  and  heat  which  are  developed 
at  a  "single  contraction  become  much  more  obvious  during 
a  tetanic  contraction. 

The  voluntary  contractions  by  which  we  execute  the 
various  movements  of  our  body  are  in  reality,  in  at  all 
events  nearly  all  cases,  tetanic  contractions,  however  short 


354  ELEMENTARY  PHYSIOLOGY  l£ss 

they  may  appear  to  be.  Thus,  when  we  contract  one  of 
our  muscles  by  an  effort  of  the  will  it  appears  that  a  series 
of  impulses  is  sent  out  in  rapid  succession  from  the  spinal 
cord,  perhaps  at  the  rate  of  twelve  or  more  in  a  second, 
to  throw  the  muscle  into  prolonged  contraction.  By  this 
means  our  control  of  the  resulting  movement  is  far  greater 
than  it  would  be  if  we  were  only  able  to  execute  single, 
short,  and  sudden  contractions,  such  as  result  from  sending 
a  single  impulse  along  the  nerve  going  to  the  muscle. 

8.  The  Various  Kinds  of  Muscles.  —  Muscles  may  be 
conveniently  divided  into  two  groups,  according  to  the 
manner  in  which  the  ends  of  their  fibres  are  fastened  ; 
into  muscles  not  attached  to  solid  levers,  and  muscles 
attached  to  solid  levers. 

Muscles  not  attached  to  Solid  Levers.  —  Under  this  head 
come  the  muscles  which  are  appropriately  called  hollow 
muscles,  inasmuch  as  they  inclose  a  cavity  or  surround  a 
space ;  and  their  contraction  lessens  the  capacity  of  that 
cavity,  or  the  extent  of  that  space. 

The  muscular  fibres  of  the  heart,  of  the  blood-vessels, 
of  the  lymphatic  vessels,  of  the  alimentary  canal,  of  the 
urinary  bladder,  of  the  ducts  of  the  glands,  of  the  iris  of 
the  eye,  are  so  arranged  as  to  form  hollow  muscles. 

In  the  heart  the  muscular  fibres,  which,  though  peculiar, 
are  striated,  are  arranged  in  an  exceedingly  complex  manner 
round  the  several  cavities,  and  they  contract,  as  we  have 
seen,  in  a  definite  order. 

The  iris  of  the  eye  is  like  a  curtain,  in  the  middle  of 
which  is  a  circular  hole.  The  muscular  fibres  are  of  the 
smooth  or  unstriated  kind  (see  p.  310),  and  they  are  dis- 
posed in  two  sets  :  one  set  radiating  from  the  edges  of  the 
hole  to  the  circumference  of  the  curtain  ;  and  the  other 
set   arranged    in   circles,  concentrically  with    the   aperture. 


vin  THE   VARIOUS   KINDS   OF   MUSCLES  325 

The  muscular  fibres  of  each  set  contract  suddenly  and 
together,  the  radiating  fibres  necessarily  enlarging  the  hole, 
the  circular  fibres  diminishing  it. 

In  the  alimentary  canal  the  muscular  fibres  are  also  of 
the  unstriated  kind,  and  they  are  disposed  in  two  layers, 
one  set  of  fibres  being  arranged  parallel  with  the  length  of 
the  intestines,  while  the  others  are  disposed  circularly,  or 
rather  at  right  angles  to  the  former. 

As  has  been  stated  above  (p.  278),  the  contraction  of 
these  muscular  fibres  is  successive ;  that  is  to  say,  all  the 
muscular  fibres,  in  a  given  length  of  the  intestines,  do  not 
contract  at  once,  but  those  at  one  end  contract  first,  and 
the  others  follow  them  until  the  whole  series  have  con- 
tracted. As  the  order  of  contraction  is,  naturally,  always 
the  same,  from  the  upper  towards  the  lower  end,  the  effect 
of  this  peristaltic  contraction  is,  as  we  have  seen,  to  force 
any  matter  contained  in  the  alimentary  canal  from  its 
upper  towards  its  lower  extremity.  The  muscles  of  the 
walls  of  the  ducts  of  the  glands  have  a  substantially  similar 
arrangement.  In  these  cases  the  contraction  of  each  fibre 
is  less  sudden  and  lasts  longer  than  in  the  case  of  the  heart. 

Muscles  attached  to  Definite  Levers.  —  The  great  majority 
of  the  muscles  in  the  body  are  attached  to  distinct  levers, 
formed  by  the  bones.  In  such  bones  as  are  ordinarily 
employed  as  levers,  the  osseous  tissue  is  arranged  in  the 
form *of  a  shaft  (Fig.  97,  d),  formed  of  a  very  dense  and 
compact  osseous  matter,  but  often  containing  a  great  central 
cavity  (t>),  which  is  filled  with  a  very  delicate  vascular  and 
fibrous  tissue  loaded  with  fat  called  marrow.  Towards  the 
two  ends  of  the  bone,  the  compact  matter  of  the  shaft  thins 
out,  and  is  replaced  by  a  much  thicker  but  looser  sponge- 
work  of  bony  plates  and  fibres,  which  is  termed  the  can- 
cellous or  spongy  tissue  of  the  bone.      The  surface  even 


ELEMENTARY   PHYSIOLOGY 


in 


Fig.  97.  —  Longitudinal 
Section  of  the  Shaft 
of    a  Human   Femur 
or  Thigh-bone. 
a,  the  head,  which  ar- 
ticulates   with     the    hip- 
bone;   b,    the   medullary 
cavity,  and  d,  the  dense 
bony    substance     of    the 
shaft;    c,  the  part  which 
enters  into  the  knee-joint, 
articulating  with  the  shin- 
bone,  or  tibia 


of  this  part,  however,  is  still  formed  by 
a  thin  sheet  of  denser  bone. 

At  least  one  end  of  each  of  these 
bony  levers  is  fashioned  into  a  smooth, 
articular  surface,  covered  with  cartilage, 
which  enables  the  relatively  fixed  end  of 
the  bone  to  play  upon  the  corresponding 
surface  of  some  other  bone,  with  which 
it  is  said  to  be  articulated  (see  p.  345), 
or,  contrariwise,  allows  the  other  bone 
to  move  upon  it. 

It  is  one  or  other  of  these  extremities 
which  plays  the  part  of  fulcrum  when  the 
bone  is  in  use  as  a  lever. 

Thus,  in  the  accompanying  figure  (Fig. 
98)  of  the  bones  of  the  upper  extremity, 
with  the  attachments  of  the  biceps  mus- 
cle to  the  shoulder-blade  and  to  one  of 
the  two  bones  of  the  fore-arm  called  the 
radius,  P  indicates  the  point  of  action 
of  the  power  (the  contracting  muscle) 
upon  the  radius. 

It  usually  happens  that  the  bone  to 
which  one  end  of  a  muscle  is  attached 
is  absolutely  or  relatively  stationary ; 
while  that  to  which  the  other  is  fixed  is 
movable.  In  this  case,  the  attachment 
to  the  stationary  bone  is  termed  the  ori- 
gin, that  to  the  movable  bone  the  inser- 
tion, of  the  muscle. 

The  fibres  of  muscles  are  sometimes 
fixed  directly  into  the  parts  which  serve 
as    their    origins    and     insertions ;     but 


nn  THE   VARIOUS   KINDS   OF   MUSCLES  327 

more  commonly  strong  cords  or  bands  of  fibrous  tissue, 
called  tendons,  are  interposed  between  the  muscle  proper 
and  its  place  of  origin  or  insertion.  When  the  tendons  play 
over  hard  surfaces,  it  is  usual  for  them  to  be  separated  from 
these  surfaces  by  sacs  containing  fluid,  which  are  called 
bursce  ;  or  even  to  be  invested  by  synovial  sheaths,  i.e.  quite 
covered  for  some  distance  by  a  bag  forming  a  double  sheath, 
very  much  in  the  same  way  that  the  bag  of  the  pleura  covers 
the  lunar  and  the  chest-wall. 


Fig.  98. — The  Bones  of  the  Upper  Extremity,  with  the  Biceps  Muscle. 

The  two  tendons  by  which  this  muscle  is  attached  to  the  scapula  are  seen  at  a 
P  indicates  the  attachment  of  the  muscle  to  the  radius,  and  hence  the  point  of  action 
of  the  power;  F,  the  fulcrum,  the  lower  end  of  the  humerus,  on  which  the  upper  end 
of  the  radius  (together  with  the  ulna)  moves;   \V,  the  weight  (of  the  hand). 


Usually,  the  direction  of  the  axis  of  a  muscle  is  that  of  a 
straight  line  joining  its  origin  and  its  insertion.  But  in  some 
muscles,  as  the  superior  oblique  muscle  of  the  eye,  the 
tendon  passes  over  a  pulley  formed  by  ligament,  and  com- 
pletely changes  its  direction  before  reaching  its  insertion. 
(See  p.  440.) 

Again,  there  are  muscles  which  are  fleshy  at  each  end, 
and  have  a  tendon  in  the  middle.     Such  muscles  are  called 


328 


ELEMENTARY   PHYSIOLOGY 


digastric,  or  two-bellied.  In  the  curious  muscle  which  pulls 
down  the  lower  jaw,  and  especially  receives  this  name  of 
digastric,  the  middle  tendon  runs  through  a  pulley  connected 
with  the  hyoid  bone ;  and  the  muscle,  which  passes  down- 
wards and  forwards  from  the  skull  to  this  pulley,  after  trav- 
ersing  it,    runs    upwards   and    forwards   to   the   lower  jaw 

(Fig-  99)- 
9.    The  Structure  of  Bone.  —  A  fresh  long  bone,  such  as 

the  femur  and  humerus  of  a  rabbit,  from  which  the  attached 

muscles,  tendons,  and  ligaments  have  been  carefully  cleaned 


Fig.  99. —  The  Course  of  the  Digastric  Muscle. 

D,  its  posterior  belly;   D' ,  its  anterior  belly;  between  the  two  is  the  tendon  passing 
through  its  pulley  connected  with  Hy,  the  hyoid  bone. 


away,  but  the  surface  of  which  has  not  been  scraped  or 
otherwise  injured,  is  an  excellent  subject  for  the  study  of 
bone.  It  is  a  hard,  tough  body,  which  is  flexible  and  highly 
elastic  within  narrow  limits,  but  readily  breaks,  with  a  clean 
fracture,  if  it  is  pressed  too  far.  The  two  articular  ends  are 
coated  by  a  layer  of  cartilage,  which  is  thickest  in  the  middle. 
Where  the  margins  of  the  cartilage  thin  out,  a  layer  of  vas- 
cular connective  tissue  commences,  and,  extending  over  the 
whole  shaft,  to  the  surface  of  which  it  is  closely  adherent, 
constitutes  the  periosteum.  If  the  bone  is  macerated  for 
some  time  in  water,  the  periosteum  may  be  stripped  off  in 


vin  THE   STRUCTURE   OF   BONE  329 

shreds  with  the  forceps.  Filaments  pass  from  its  inner  sur- 
face into  the  interior  of  the  bone.  If  the  shaft  is  broken 
across  it  will  be  found  to  contain  a  spacious  medullary  cavity 
(Fig.  97,  b)  filled  by  a  reddish,  highly  vascular  mass  of  con- 
nective tissue,  abounding  in  fat  cells,  called  the  medulla  or 
marrow ;  and  a  longitudinal  section  shows  that  this  medul- 
lary cavity  extends  through  the  shaft,  but  in  the  articular 
ends  becomes  subdivided  by  bony  partitions  and  breaks  up 
into  smaller  cavities,  like  the  areola;  of  connective  tissue. 
These  cavities  are  termed  cancelli,  and  the  ends  of  the  bone 
are  said  to  have  a  cancellated  structure.-  The  wails  of  the 
medullary  cavity  in  the  shaft  are  very  dense  and  exhibit  no 
cancel]  i,  and  appear  at  first  to  be  solid  throughout.  But  on 
examining  them  carefully  with  a  magnifying  glass  it  will  be 
seen  that  they  are  traversed  by  a  meshwork  of  narrow  canals, 
varying  in  diameter  from  20^  to  ioo/x.  or  more.  The  long 
dimensions  of  the  meshes  lie  parallel  with  the  axis  of  the 
shaft.  These  are  the  Haversian  canals.  This  system  of 
Haversian  canals  opens  by  short  communicating  branches 
on  the  one  hand  upon  the  periosteal  and  on  the  other  upon 
the  medullary  surface  of  the  wall  of  the  shaft  ;  and  in  a  fresh 
bone,  minute  vascular  prolongations  of  the  periosteum  and 
of  the  medulla,  respectively,  may  be  seen  to  pass  into  the 
communicating  canals  and  become  continuous  with  the 
likewise  vascular  contents  of  the  Haversian  canals.  More- 
over, at  one  part  of  the  shaft  there  is  a  larger  canal,  through 
which  the  vessels  which  supply  the  medulla  pass.  This  is 
the  so-called  nutritive  foramen  of  the  bone.  At  the  two 
ends  of  the  bone  the  cavities  of  the  Haversian  canals  open 
into  those  of  the  cancelli ;  and  the  vascular  substance  which 
fills  the  latter  thus  further  connects  the  vascular  contents  of 
the  Haversian  canals  with  the  medulla. 

Thus  the  bone  may  be  regarded  as  composed  of  (i)   an 


330  ELEMENTARY   PHYSIOLOGY  less. 

interna],  thick  cylinder  of  vascular  medulla  ;  (ii)  an  external, 
hollow,  thin,  cylindrical  sheath  of  vascular  periosteum,  com- 
pleted at  each  end  by  a  plate  of  articular  cartilage  ;  (hi)  of 
a  fine,  regular,  long-meshed  vascular  network,  which  connects 
the  sides  of  the  medullary  cylinder  with  the  periosteal  sheath 
of  the  shaft ;  (iv)  of  a  coarse,  irregular,  vascular  meshwork 
occupying  at  each  end  the  space  between  the  medullary 
cylinder  and  the  plate  of  articular  cartilage,  and  connected 
with  the  periosteum  of  the  lateral  parts  of  the  articular  end  ; 
and  (v)  of  the  hard,  perfect,  osseous  tissue  which  fills  the 
meshes  of  these  two  networks.  Such  is  the  general  structure 
of  all  long  bones  with  cartilaginous  ends,  though  some,  as 
the  ribs,  possess  no  wide  medullary  cavity,  but  are  simply 
cancellated  in  the  interior.  In  some  very  small  bones  even 
the  cancelli  are  wanting.  And  there  are  many  bones  which 
have  no  connection  with  cartilage  at  all. 

If  a  bone  is  exposed  to  a  red  heat  for  some  time  in  a 
closed  vessel  nothing  remains  but  a  mass  of  white  "  bone- 
earth,"  which  has  the  general  form  of  the  bone,  but  is  very 
brittle  and  easily  reduced  to  powder.  It  consists  almost 
entirely  of  calcium  phosphate  and  carbonate.  On  the 
other  hand,  if  the  bone  is  digested  in  dilute  hydrochloric 
acid  for  some  time  the  calcareous  salts  are  dissolved  out, 
and  a  soft,  flexible  substance  is  left,  which  has  the  exact 
form  of  the  bone,  but  is  much  lighter.  If  this  is  boiled  for 
a  long  time  it  will  yield  much  gelatine,  and  only  a  small 
residue  will  be  left.  Osseous  tissue  therefore  consists  essen- 
tially of  an  animal  matter  impregnated  with  calcium  salts, 
the  animal  matter  being  collagenous,  like  connective  tissue. 

A  sufficiently  thin  longitudinal  section,  made  by  grinding 
down  part  of  the  wall  of  the  medullary  cavity  of  a  bone  — 
which  has  been  well  macerated  in  water  and  then  thoroughly 
dried  —  if  viewed  as  a  transparent  object  with  a  magnify- 


viii  THE   STRUCTURE   OF   BONE  331 

ing  glass,  shows  a  series  of  lines,  with  dark  enlargements  at 
intervals,  running  parallel  with  the  Haversian  canals.  If 
the  section,  instead  of  being  longitudinal,  were  made 
transversely  to  the  shaft,  and  therefore  cutting  through  the 
majority  of  the  Haversian  canals  at  right  angles  to  their 
length,  similar  lines  and  dark  spots  would  be  seen  to  form 
concentric  circles  at  regular  intervals  round  each  Haversian 
canal  (Fig.  100).  The  hard  bony  tissue  appears  therefore 
to  be  composed  of  lamellae,  which  are  disposed  concentri- 
cally around  the  Haversian  canals ;  and  a  Haversian  canal 
with  the  concentric  lamellae  belonging  to  it  forms  what  is 
called  a  Haversian  system.  The  soft  substance  from  which 
the  bone-earth  has  been  extracted  is  similarly  lamellated, 
and  here  and  there  presents  fibres  which  may  be  traced  into 
the  fibrous  substance  of  the  periosteum. 

If  a  thin  section  of  dry  bone  is  examined  with  the  micro- 
scope (Fig.  101),  by  transmitted  light,  each  dark  spot  is 
seen  to  be  a  black  body  (of  an  average  diameter  of  about 
15/x)  with  an  irregular  jagged  outline,  and  proceeding  from 
it  are  numerous  fine  dark  lines  which  ramify  in  the  sur- 
rounding matrix  and  unite  with  similar  branched  lines  from 
adjacent  black  bodies.  The  matrix  itself  has  a  somewhat 
granular  aspect.  In  a  transverse  section  these  black  bodies 
are  rounded  or  oval  in  form,  but  in  a  longitudinal  section 
they  appear  almost  spindle-shaped  ;  that  is  to  say,  they  are 
lenticular  or  lens-shaped  ;  but  flattened  as  it  were  between 
the  adjacent  layers  of  the  matrix.  Examined  by  reflected 
light  the  same  bodies  look  white  and  glistening ;  and  if  the 
section,  instead  of  being  examined  dry,  be  boiled  in  water 
or  soaked  in  strong  alcohol,  and  brought  under  the  micro- 
scope while  still  wet,  the  black  bodies  with  their  branching 
lines  will  be  found  to  have  almost  disappeared,  only  faint 
outlines  of  them   being   left.     At   the   same   time  minute 


332  ELEMENTARY   PHYSIOLOGY  less 

bubbles   of  air  will   have   escaped   from   the   section.     The 
black  bodies  seen  in  the  dry  bone  are  in  fact  "  lacunae," 

a 


W^ptZJ-- 


i 


5  $Wr*JNi*W*    ^*F* 


«|T4  W&  '    WAS 

Fig.  ioo.  —  Transverse  Section  of  Compact  Bone 

a,  lamellae  concentric  with  the  external  surface;  />,  lamella;  concentric  with  the 
.medullary  surface;  c,  section  of  Haversian  canals;  <:',  section  of  a  Haversian  canal 
just  dividing  into  two;  d,  intersystemic  lamella;.     Low  magnifying  power. 

i.e.  gaps,  or  holes  in  the  solid  matrix,  appearing  black  by 
transmitted  light  and  white  by  reflected  light,  because  they 


THE   STRUCTURE   OE   BONE 


333 


are  filled  with  air;  and  the  dark  branched  lines  are 
similarly  minute  canals,  "  canaliculi,"  also  filled  with  air- 
bubbles,  drawn  out,  so  to  speak,  into  lines,  also  hollowed 
out  of  the  solid  matrix,  and  placing  one  lacuna  in  communi- 
cation with  another.  In  each  Haversian  system  the  cana- 
liculi and  the  lacunce  of  the  innermost  layer,  or  that  nearest 


Fig.  ioi. — Transverse  Section  of  Bone,  highly  magnified  (300  diameters) 
H,  Haversian  canals;  /,  lacunae  with  canaliculi. 


the  Haversian  canal,  communicate  with  it,  while  the  cana- 
liculi and  the  lacuna  of  the  outermost  layer  communicate 
only  with  those  of  the  next  inner  layer.  Hence  the  lactone 
and  canaliculi  compose  a  meshwork  of  canals,  which  is 
peculiar  to  each  Haversian  system,  and  by  which  the  nutri- 
tive plasma  exuded  from  the  vessels  in  the  canal  of  that 


334  ELEMENTARY   PHYSIOLOGY  less. 

system  irrigates  all  the  layers  of  bone  which  belong  to  the 
system. 

A  very  thin  section  of  perfectly  fresh  bone  exhibits  no 
dark  bodies,  inasmuch  as  the  lacunae  and  canaliculi  con- 
tain no  air,  but  are  permeated  with  the  nutritive  fluid. 
Each  lacuna,  moreover,  at  all  events  in  young  bone,  con- 
tains a  nucleated  cell,  which  is  altogether  similar  in  essen- 
tial character  to  a  connective-tissue  or  cartilage  corpuscle, 
and  if  the  term  were  not  already  misused  might  be  called 
a  bone  corpuscle.  In  fact,  in  ultimate  analysis  the  essen- 
tial character  of  bone  shows  itself  to  be  this  :  that  it  is 
a  tissue  analogous  to  cartilage  and  connective  tissue  in  so 
far  as  it  consists  of  cells  separated  by  much  intercellular 
substance ;  and  that  it  differs  from  them  mainly  in  the  fact 
that  calcareous  matter  is  deposited  in  and  associated  with 
the  intercellular  substance  in  such  a  way  as  to  leave  minute 
uncalcified  passages  (the  canaliculi),  which  open  into  the 
larger  uncalcified  intervals  (the  lacuna)  in  the  neighbour- 
hood of  the  cells. 

The  function  of  these  passages  is  doubtless  to  allow  of  a 
more  thorough  permeation  of  the  calcified  tissue  by  the  nutri- 
tive fluids  than  could  take  place  if  the  calcareous  deposit 
were  continuous,  and  it  is  probable  that,  in  an  ordinary  bone, 
there  is  no  particle  i/n  square  which  is  not  thus  brought 
within  reach  of  a  minute  streamlet  of  nutritive  plasma. 

This  circumstance  enables  us  to  understand  that  which 
one  would  hardly  suspect  from  the  appearance  of  a  bone, 
namely,  that,  throughout  life,  or,  at  all  events,  in  early  life, 
its  tissue  is  the  seat  of  an  extremely  active  vital  process. 
The  permanence  and  apparent  passivity  of  the  bone  are 
merely  the  algebraical  summation  of  the  contrary  processes 
of  destruction  and  reproduction  which  are  going  on  in  it. 

If  a  young  pig  is  fed  with  madder,  its  bones  will  be  found 


viii  THE   DEVELOPMENT   OF   BONE  335 

after  a  time  to  be  dyed  red.  The  madder  dye,  in  fact,  get- 
ting into  the  blood,  permanently  dyes  the  tissue  with  which 
it  meets  in  its  course  through  the  bones.  But  if  the  pig  is 
fed  for  a  time  with  madder,  and  is  then  deprived  of  it,  the 
amount  of  colour  to  be  found  in  the  bones  depends  on  the 
time  which  elapses  before  the  pig  is  killed.  And  it  is  not 
that  the  colouring  matter  is  merely,  as  it  were,  washed  out ; 
the  dye  is  permanent,  but  the  bones  nevertheless  become 
parti-coloured.  In  the  shaft  of  a  long  bone,  for  instance,  a 
certain  time  after  feeding  with  madder,  a  deep  red  layer 
of  bone  in  the  middle  of  the  thickness  of  its  wall  will  be 
found  to  have  colourless  bone  on  its  medullary  and  on  its 
periosteal  face.  And  the  longer  the  time  which  has  elapsed 
since  the  feeding  with  madder,  the  more  completely  will  the 
deep  red  bone  be  replaced  and  covered  up  by  colourless  bone. 
10.  The  Development  of  Bone.  —  Careful  inspection  of  a 
transverse  section  of  the  wall  of  the  shaft  of  a  long  bone  is  by 
itself  sufficient  to  show  that  bone  is  constantly  being  formed 
and  as  constantly  being  removed.  Such  a  section  exhibits, 
as  has  been  said,  a  number  of  Haversian  canals  surrounded 
by  circular  zones  formed  of  concentric  layers  of  bone.  But 
interspersed  between  these  there  lie  larger  and  smaller  seg- 
ments of  zones  formed  of  similar  concentrically  curved  par- 
allel lamellae,  the  so-called  intersystemic  lamellae  (Fig.  100,  d), 
which  have  evidently  at  one  time  formed  parts  of  complete 
Haversian  systems,  but  which  have  been  partially  destroyed 
and  replaced  by  new  systems.  In  fact  the  formation  of  new 
bone  is  constantly  taking  place  :  (i)  at  the  surface  in  con- 
tact with  the  periosteum;  (ii)  at  the  surface  in  contact 
with  cartilage;  and  (iii)  at  the  surface  in  contact  with  the 
medulla  and  its  prolongations  in  the  cancelli  and  the  Haver- 
sian canals  ;  and  the  bone  thus  formed  is  after  a  time  de- 
stroyed and  replaced  by  new  growths. 


336  ELEMENTARY   PHYSIOLOGY  less. 

To  understand  this  we  must  study  the  origin  of  osseous 
tissue.  At  a  certain  period  of  embryonic  life  there  is  no  bone 
in  any  part  of  the  body.  Nevertheless,  the  greater  number 
of  the  "  bones,"  for  example  the  vertebras,  the  ribs,  the  limb 
bones,  and  some  of  the  cranial  and  facial  bones,  exist  in  a 
morphological  sense,  inasmuch  that  cartilages  having  the 
general  form  of  such  bones  exist  in  the  places  of  the  future 
bones.  In  the  place  of  the  humerus  and  the  femur,  for 
example,  there  are  rods  of  pure  cartilage,  which  are,  so  to 
speak,  small,  rough  models  of  the  humerus  and  femur  of 
the  adult.  When  the  process  of  bone  formation  com- 
mences, slight  opaque  spots,  termed  centres  of  ossification, 
make  their  appearance  in  the  substance  of  the  cartilage, 
the  opacity  being  due  to  the  deposit  of  calcareous  salts 
at  these  points. 

Microscopic  examination  shows  that  the  calcareous  salts 
are  deposited  in  the  intercellular  substance,  which,  therefore, 
is  converted  into  a  sort  of  bone,  in  which  the  lacunae  are 
represented  by  the  cavities  of  the  cartilage  corpuscles. 
These  calcareous  salts  must  reach  the  centres  of  ossification 
dissolved  in  the  plasma  which  is  exuded  from  the  perichon- 
drial  vessels  and  permeates  the  intercellular  substance. 

In  the  cartilaginous  rudiment  of  a  long  bone  three  such 
centres  of  ossification  usually  make  their  appearance,  one 
in  the  centre  of  the  shaft  and  one  in  each  end.  Supposing 
these  centres  to  be  formed  at  the  same  time  (which  may 
not,  however,  be  the  case),  what  we  have  to  start  from  is  a 
rudiment  or  model  in  cartilage  of  the  future  bone,  converted 
at  three  points  into  calcified  cartilage  ;  that  is  to  say  there 
are  a  central  nodule  (diaphysis)  and  two  terminal  nodules 
(epiphyses) .  If  the  deposit  were  to  spread  from  the  three 
centres  until  the  three  nodules  united,  the  result  would  be 
a  calcified  cartilage  in  place  of  the  formative  cartilage. 


viii  THE   DEVELOPMENT  OF   BONE  337 

As  a  matter  of  fact,  the  deposit  does  spread  through  the 
rudiment  from  each  centre  outwards  so  long  as  the  bone  is 
growing.  But  the  cartilage  between  the  diaphysis  and  epi- 
physes and  beyond  the  ends,  of  the  epiphyses  also  grows 
and  increases  with  the  general  growth  of  the  bone.  That 
beyond  the  epiphysial  ossification  remains  throughout  life  as 
articular  cartilage,  while  that  between  the  epiphysial  and 
diaphysial  ossifications  is  gradually  encroached  upon  by 
these  and  finally  obliterated. 

If  this  were  all,  the  adult  bone  would  consist  of  calcified 
cartilage  (Fig.  102,  c)  tipped  at  the  ends  with  cartilage  which 
remained  uncalcified.  But  this  is  not  all ;  such  a  mass  of 
calcified  cartilage  is  not  a  true  bone. 

Very  soon  after  the  ossific  centres  have  made  their  appear- 
ance, there  grow  into  them  vascular  processes  of  the  peri- 
chondrium, or  membrane  of  connective  tissue  containing 
blood-vessels  that  surrounds  the  cartilage  and  later  is  called 
the  periosteum.  These  processes  make  room  for  themselves 
by,  in  some  way,  causing  the  destruction  and  absorption  of 
the  calcified  cartilage,  thus  giving  rise  to  large  irregular 
spaces  or  areolae,  which  they  occupy.  The  processes  con- 
sist of  blood-vessels  surrounded  by  a  peculiar  form  of 
connective  tissue,  characterised  by  the  presence  of  large 
nucleated  cells  called  osteoblasts. 

No  sooner  have  these  processes  hollowed  out  the  areola? 
in  the  calcified  cartilage  than  they  begin  to  line  them  with 
layers  of  true  bone  (c.d),  the  matrix  of  the  connective  tis- 
sue of  the  processes  being  calcified  in  such  a  way  as  to  leave 
spaces,  in  which  some  of  the  cells  or  osteoblasts  remain  im- 
bedded, fine  branching  canals  being  left  in  the  matrix,  or 
being  subsequently  formed  in  it.  In  other  words,  layers  of 
true  bone,  with  lacunas  containing  nucleated  cells  and  with 
branched  canaliculi,  are  thus  constructed  as  a  lining  to  the 
z 


33S 


ELEMENTARY   PHYSIOLOGY 


spaces  hollowed  out  of  the  calcified  cartilage.  None  of  the 
spaces,  however,  are  completely  filled  up,  and  there  are  no 
signs  of  regular  Haversian  systems  with  canals  and  concen- 
tric laminae.  The  calcified  cartilage 
is  simply  replaced  by  a  loose  open 
network  of  spongy  bone,  in  the  thick- 
ness of  the  bars  of  which  may  be 
seen  the  remains  of  the  calcified 
cartilage,  and  the  cavities  of  which 
are  filled  with  blood-vessels  and  deli- 
cate connective  tissue,  that  is,  with 
marrow. 

Meanwhile  the  perichondrium  or 
periosteum,  in  addition  to  sending 
in  these  processes,  which  thus  con- 
vert the  calcified  cartilage  into 
spongy  but  true  bone,  also  deposits 
layers  of  somewhat  denser  but  still 
spongy  bone  on  the  outside  of  the 
changed  and  changing  ossific  cen- 
tre, in  the  form  of  a  cylinder  (p.b), 
which    grows   in   thickness   by   the 

Fig.  102.  —  Longitudinal  Sec-        it.-  r  1  •..  r 

■noNOF  Ossifying  Humerus    addition  of  new  layers  On  its  Surface, 

......  immediately  under  the  periosteum, 

c,  the  original  primitive  carti-  J  1 

lage,  calcined  in  its  deeper  por-     ancl    in    length    by    the    extension    of 
tion;  c.b,   spongy   bone  arising  °  ■* 

from  ossification   of  calcified   these  cylindrical  layers  upwards  and 

cartilage;  this  has  already  been  J 

absorbed   and  replaced  by  me-     downwards.     The  "  periosteal "  bone, 

as  this  is  called,  is  also  true  bone, 
the  deposition  of  calcic  salts  tak- 
ing place  in  the  matrix  around  the 

osteoblasts  in  such  a  way  as  to  leave  lacunae  and  canaliculi. 
Very  soon  after  this  sheath  of  periosteal  bone  has  made 

its  appearance,  the  spongy  bone  first  formed  in  the  interior 


dullaat  »i:  p.b.  bone  formed  by 
the  periosteum;  it  is  seen  ex- 
tending as  a  thin  sheet  upwards 
and  downwards  outside  the  carti- 
lage.    (Magnified  7  diameters.) 


vni  THE   DEVELOPMENT  OF   BONE  339 

is  itself  absorbed  by  the  same  vascular  processes  which 
formed  it,  so  that  soon  what  was  at  first  the  centre  of  ossifi- 
cation, after  passing  from  simple  cartilage  to  calcified  carti- 
lage, and  so  to  spongy  bone,  is  resolved  into  marrow  or 
medulla  (m),  that  is,  into  vascular  connective  tissue  richly 
loaded  with  fat. 

The  cartilage  at  each  end  of  the  medulla  continues  to 
grow  in  length  and  thickness,  and  to  be  successively  con- 
verted, first,  into  calcified  cartilage,  and  then  into  spongy 
bone  at  its  end  nearest  the  medulla.  The  medulla  also 
increases  rapidly  in  length,  encroaching  more  and  more 
upon  the  spongy  bone. 

The  whole  is  surrounded  by  the  ring  or  cylinder  of  perios- 
teal bone  just  described,  which  also  grows  in  thickness  and 
length  and  assumes  the  form  of  a  long,  narrow  dice-box, 
with  narrow  but  thicker  walls  in  the  middle,  and  with  wider 
but  thinner  walls  at  each  end.  The  middle  of  the  cylinder 
is  occupied  by  medulla  alone,  but  each  end  is,  as  it  were, 
plugged  by  a  disc  of  cartilage  undergoing  conversion  into  cal- 
cified cartilage  (c),  then  into  spongy  bone  (c.b),  and  finally 
into  medulla  (m). 

As  the  developing  bone  grows,  the  discs  get  farther  and 
farther  apart,  and  the  medulla  grows  longer  until  the  two 
ends  of  the  diaphysis  meet  the  epiphyses,  and  unite  with 
them.  The  whole  disc  thus  becomes  at  last  spongy  bone, 
continuous  with  the  similar  spongy  bone  into  which  the 
epiphysis  is  converted,  and  forms  the  spongy  bone  existing 
at  the  ends  of  the  long  bones  ;  all  that  remains  of  the  calci- 
fied cartilage  is  an  exceedingly  thin  layer  just  below  the 
articular  cartilage  at  either  end  of  the  bone. 

Thus,  though  the  primitive  cartilage  serves  as  the  model 
of  the  future  bone,  a  great  deal  of  the  bone,  namely,  the 
dense,  compact  bone  which  forms  the  shaft  and  is  con- 


340  ELEMENTARY   PHYSIOLOGY  less 

tinued  as  a  shell  over  the  two  ends,  does  not  come  from  the 
cartilage  at  all  but  is  deposited  by  the  periosteum ;  the 
spongy  bone  at  each  end  is  the  only  part  that  is  formed  in 
the  cartilage,  and  even  in  that,  as  we  have  seen,  there  are 
no  remains  of  the  cartilage  itself. 

Moreover,  the  bone  even  thus  formed  is  subject  to  inces- 
sant change.  The  periosteal  bone  is  at  first  spongy  and 
slight  in  texture,  and  exhibits  no  true  Haversian  systems. 
Little  by  little,  spaces  are  scooped  out  in  it  by  vascular  pro- 
cesses of  the  periosteum  on  the  outside  and  of  the  medulla 
on  the  inside,  like  those  which  formed  it ;  and  such  a  space 
when  formed  is  in  turn  filled  up  in  a  solid  fashion  by  layers 
of  bone  deposited  in  a  regular  way  -as  concentric  lamellae 
round  the  blood-vessels  of  the  process,  which  in  the  end 
remains  as  the  blood-vessel  of  the  Haversian  canal,  in  the 
centre  of  the  Haversian  system  thus  deposited.  And  indeed 
similar  processes  of  absorption  and  fresh  formation  go  on 
certainly  while  the  bone  is  increasing  in  size,  and  probably 
also  for  some  time  afterwards. 

A  good  many  bones,  such  as  the  frontal  and  parietal  bones 
of  the  skull,  have  no  cartilaginous  precursors.  The  roof  of 
the  skull  of  an  embryo  is  formed  of  a  membrane  of  con- 
nective tissue,  and  in  this  each  of  the  bones  commences  as 
a  calcification  of  that  part  of  the  connective  tissue  which 
occupies  the  place  of  the  centre  of  the  future  bone.  The 
calcification  radiates  from  this  centre  outwards,  so  that  it 
soon  has  the  form  of  a  thin  plate,  the  margins  of  which  are, 
as  it  were,  frayed  out  in  filaments.  The  vascular  connective 
tissue  which  incloses  the  plate  becomes  its  periosteum,  and 
plays  the  same  part  in  relation  to  the  growing  bone  as  the 
periosteum  of  cartilage  bone  does  to  it.  As  the  plate  grows 
thicker,  medullary  processes  burrow  into  it  and  give  rise  to 
canr.elU  and  Haversian  systems. 


vui  THE   MECHANICS   OK    MOTION.      LEVERS  54. 

11.  The  Mechanics  of  Motion.  Levers. — To  understand 
the  action  of  the  bones,  as  levers,  properly,  it  is  necessary  to 
possess  a  knowledge  of  the  different  kinds  of  levers  and  be 
able  to  refer  the  various  combinations  of  the  bones  to  their 
appropriate  lever-classes. 

A  lever  is  a  rigid  bar,  one  part  of  which  is  absolutely  or 
relatively  fixed,  while  the  rest  is  free  to  move.  Some  one 
point  of  the  movable  part  of  the  lever  is  set  in  motion 
by  a  force,  in  order  to  communicate  more  or  less  of  that 
motion  to  another  point  of  the  movable  part,  which  pre- 
sents a  resistance  to  motion  in  the  shape  of  a  weight  or 
other  obstacle. 

Three  kinds  of  levers  are  enumerated  by  mechanicians, 
the  definition  of  each  kind  depending  upo  1  the  relative  posi- 
tions of  the  point  of  support,  or  fulcrum;  of  the  point 
which  bears  the  resistance,  weight,  or  other  obstacle  to  be 
overcome  by  the  force  ;  and  of  the  point  to  which  the  force, 
or  power  employed  to  overcome  the  obstacle,  is  applied. 

If  the  fulcrum  be  placed  between  the  power  and  the 
weight,  so  that,  when  the  power  sets  the  lever  in  motion, 
the  weight  and  the  power  describe  arcs,  the  concavities  of 
which  are  turned  towards  one  another,  the  lever  is  said  to  be 
of  the  first  class.      (Fig.  103,  I.) 

If  the  fulcrum  be  at  one  end,  and  the  weight  be  between 
it  and  the  power,  so  that  weight  and  power  describe  concen- 
tric arcs,  the  weight  moving  through  the  less  space  when  the 
lever  moves,  the  lever  is  said  to  be  of  the  second  class. 
(Fig.  103,  II.) 

And  if,  the  fulcrum  being  still  at  one  end,  the  power  be 
between  the  weight  and  it,  so  that,  as  in  the  former  case, 
the  power  and  weight  describe  concentric  arcs,  but  the 
power  moves  through  the  less  space,  the  lever  is  of  the 
third  class.      (Fig.  103,  III.) 


342 


ELEMENTARY    PHYSIOLOGY 


LESS 


In  the  human  body  the  following  parts  present  examples 
of  levers  of  the  first  class. 

(a)  The    skull   in   its    movements    upon    the    atlas,   as 
fulcrum. 

(b)  The  pelvis  in  its  movements  upon  the  heads  of  the 
thigh-bones,  as  fulcrum. 

(c)  The  foot,  when,  it  is  raised,  and  the  toe  tapped  on 
the  ground,  the  ankle-joint  being  fulcrum.     (Fig.  103,  I.) 


ill 


Fig.  103. 


The  upper  three  figures  represent  the  three  kinds  of  levers;  the  lower,  the  foot 
when  it  takes  the  character  of  each  kind.  — W,  weight  or  resistance;  F,  fulcrum;  P, 
power. 


The  positions  of  the  weight  and  of  the  power  are  not  given 
in  either  of  these  cases,  because  they  are  reversed  accord- 
ing to  circumstances.  Thus,  when  the  face  is  being  de- 
pressed, the  power  is  applied  in  front,  and  the  weight  to 
the  back  part,  of  the  skull  ;  but  when  the  face  is  being 
raised,  the  power  is  behind  and  the  weight  in  front.  The 
like  is  true  of  the  pelvis,  according  as  the  body  is  bent  for- 
ward, or  backward,  upon  the  legs.  Finally,  when  the  toes, 
in  the  action  of  tapping,  strike  the  ground,  the  power  is  at 
the  heel,  and  the  resistance  in  the  front  of  the  foot.  But 
when,  the  toes  are  raised  to  repeat  the  act,  the  power  is  in 


vin  THE   MECHANICS   OF   MOTION.      LEVERS  343 

front,  and  the  weight,  or  resistance,  is  at  the  heel,  being,  in 
fact,  the  inertia  and  elasticity  of  the  muscles  and  other  parts 
of  the  back  of  the  leg. 

But  in  all  these  cases,  the  lever  remains  one  of  the  first 
class,  because  the  fulcrum,  or  fixed  point  on  which  the 
lever  turns,  remains  between  the  power  and  the  weight,  or 
resistance. 

The  following  are  three  examples  of  levers  of  the  second 
class  :  — 

(a)  The  thigh-bone  of  the  leg  which  is  bent  up  towards 
the  body  and  not  used,  in  the  action  of  hopping. 

For,  in  this  case,  the  fulcrum  is  at  the  hip-joint.  The 
power  (which  may  be  assumed  to  be  furnished  by  the  thick 
muscle  J  of  the  front  of  the  thigh)  acts  upon  the  knee-cap  ; 
and  the  position  of  the  weight  is  represented  by  that  of  the 
centre  of  gravity  of  the  thigh  and  leg,  which  will  lie  some- 
where between  the  end  of  the  knee  and  the  hip. 

{b)  A  rib  when  depressed  by  the  rectus  muscle 2  of  the 
abdomen,  in  expiration. 

Here  the  fulcrum  lies  where  the  rib  is  articulated  with 
the  spine;  the  power  is  at  the  sternum  —  virtually  the 
opposite  end  of  the  rib ;  and  the  resistance  to  be  overcome 
lies  between  the  two. 

(c)  The  raising  of  the  body  upon  the  toes,  in  standing 
on  tiptoe,  and  in  the  first  stage  of  making  a  step  forward. 
(Fig.  103,  II.) 

Here  the  fulcrum  is  the  ground  on  which  the  toes  rest ; 
the  power  is  applied  by  the  muscles  of  the  calf  to  the  heel 

1  This  muscle,  called  rectus,  is  attached  above  to  the  hip-bone  and  below 
to  the  knee-cap  (Fig.  6,  2,  p.  18) .  The  latter  bone  is  connected  by  a  strong 
ligament  with  the  tibia. 

2  This  muscle  lies  in  the  front  abdominal  wall  on  each  side  of  the  middle 
line.  It  is  attached  to  the  sternum  above  and  to  the  front  of  the  pelvis 
below.     (Fig.  6,  3.) 


344  ELEMENTARY   PHYSIOLOGY  less. 

(Fig.  6,  I.)  ;  the  resistance  is  so  much  of  the  weight  of  the 
body  as  is  borne  by  the  ankle-joint  of  the  foot,  which  of 
course  lies  between  the  heel  and  the  toes. 

Three  examples  of  levers  of  the  third  class  are  — 

(a)  The  spine,  head  and  pelvis,  considered  as  a  rigid  bar, 
which  has  to  be  kept  erect  upon  the  hip-joints.     (Fig.  6.) 

Here  the  fulcrum  lies  in  the  hip-joints,  the  weight  is  high 
above  the  fulcrum,  at  the  centre  of  gravity  of  the  head  and 
trunk  ;  the  power  is  supplied  by  the  extensor  muscles  (Fig. 
6,  2)  in  the  front  of,  or  the  flexor  muscles  (Fig.  6,  II.)  at 
the  back  of,  the  thigh,  and  acts  upon  points  comparatively 
close  to  the  fulcrum. 

(£)  Flexion  of  the  forearm  upon  the  arm  by  the  biceps 
muscle,  when  a  weight  is  held  in  the  hand. 

In  this  case,  the  weight  being  in  the  hand  and  the  ful- 
crum at  the  elbow-joint,  the  power  is  applied  at  the  point  of 
attachment  of  the  tendon  of  the  biceps,  close  to  the  latter. 
(Fig.  98.) 

(c)   Extension  of  the  leg  on  the  thigh  at  the  knee-joint. 

Here  the  fulcrum  is  the  knee-joint ;  the  weight  is  at  the 
centre  of  gravity  of  the  leg  and  foot,  somewhere  between 
the  knee  and  the  foot ;  the  power  is  applied  by  the  muscles 
in  front  of  the  thigh  (Fig.  6,  2  and  Fig.  104),  through  the 
ligament  of  the  knee-cap,  or  patella,  to  the  tibia,  close  to 
the  knee-joint. 

In  studying  the  mechanism  of  the  body,  it  is  very  impor- 
tant to  recollect  that  one  and  the  same  part  of  the  body 
may  represent  each  of  the  three  kinds  of  levers,  according 
to  circumstances.  Thus,  it  has  been  seen  that  the  foot  may, 
under  some  circumstances,  represent  a  lever  of  the  first,  in 
others,  of  the  second  class.  But  it  may  become  a  lever  of 
the  third  class,  as  when  one  dances  a  weight  resting  upon 
the  toes  up  and  down,  by  moving  only  the  foot.  •  In  this 


THE  JOINTS   OF  THE   BODY 


345 


case,  the  fulcrum  is  at  the  ankle-joint,  the  weight  is  at  the 
toes,  and  the  power  is  furnished  by  the  extensor  muscles  at 
the  front  of  the  leg  (Fig.  6,  i),  which  are  inserted  between 
the  fulcrum  and  the  weight.     (Fig.  103,  III.) 


Fig.  104.  —  The   Right   Knee-joint.    The   Outer  Half  of  the  Femur  and 
Patella  sawn  away. 

fern,  femur;  pat,  patella;  til,  tibia;  fib,  fibula;  caps,  capsule  of  joint;   /,  crucial 
ligaments;  c,  semilunar  fibro-cartilages;  e,  tendon  of  extensor  muscle. 


12.  The  Joints  of  the  Body.  —  It  is  very  important  that 
the  levers  of  the  body  should  not  slip,  or  work  unevenly, 
when  their  movements  are  extensive,  and  to  this  end  they 
are  connected  together  in  such  a  manner  as  to  form  strong 
and  definitely  arranged  joints  or  articulations. 

Joints  may  be  classified  into  imperfect  and  perfect. 

(a)  Imperfect  joints  are  those  in  which  the  conjoined 
levers  (bones    or   cartilages)    present   no    smooth    surfaces 


346  ELEMENTARY   PHYSIOLOGY  less 

capable  of  rotatory  motion,  to  one  another,  but  are  connected 
by  continuous  cartilages  or  ligaments,  and  have  only  so 
much  mobility  as  is  permitted  by  the  flexibility  of  the  join- 
ing substance. 

Examples  of  such  joints  as  these  are  to  be  met  with  in 
the  vertebral  column  —  the  flat  surfaces  of  the  bodies  of  the 
vertebras  being  connected  together  by  thick  plates  of  very 
flexible  fibro-cartilage,  which  confer  upon  the  whole  column 
considerable  play  and  springiness,  and  yet  prevent  any  great 
amount  of  motion  between  the  several  vertebrae.  In  the 
pelvis  (see  Fig.  4),  the  pubic  bones  are  united  to  each 
other  in  front,  and  the  iliac  bones  to  the  sacrum  behind,  by 
fibrous  or  cartilaginous  tissue,  which  allows  of  only  a  slight 
play,  and  so  gives  the  pelvis  a  little  more  elasticity  than  it 
would  have  if  it  were  all  one  bone. 

(3)  In  all  perfect  joints,  the  opposed  bony  surfaces 
which  move  upon  one  another  are  covered  with  cartilage, 
and  between  them  is  placed  a  sort  of  sac,  which  lines  these 
cartilages,  and,  to  a  certain  extent,  forms  the  side  walls  of 
the  joint ;  and  which,  secreting  a  small  quantity  of  viscid, 
lubricating  fluid  —  the  synovial  fluid  —  is  called  a  synovial 
membrane. 

The  opposed  surfaces  of  these  articular  cartilages,  as 
they  are  called,  may  be  spheroidal,  cylindrical,  or  pulley- 
shaped  ;  and  the  convexities  of  the  one  answer,  more  or  less 
completely,  to  the  concavities  of  the  other. 

Sometimes,  the  two  articular  cartilages  do  not  come 
directly  into  contact,  but  are  separated  by  independent 
plates  of  cartilage,  which  are  termed  inter- articular.  The 
opposite  faces  of  these  inter-articular  cartilages  are  fitted  to 
receive  the  faces  of  the  proper  articular  cartilages. 

While  these  co-adapted  surfaces  and  synovial  membranes 
provide  for  the  free  mobility  of  the  bones  entering  into  a 


THE  JOINTS   OF  THE   BODY 


347 


joint,  the  nature  and  extent  of  their  motion  is  defined,  partly 
by  the  forms  of  the  articular  surfaces,  and  partly  by  the 
disposition  of  the  ligaments,  or  firm,  fibrous  cords  which 
pass  from  one  bone  to  the  other. 

As  respects  the  nature  of  the  articular  surfaces,  joints 
may  be  what  are  called  ball-and-socket  joints,  when  the 
spheroidal  surface  furnished  by  one   bone  plays  in  a  cup 


Fig.  105. — A  Section  of  the  Hip- joint  taken  through  the  Acetabulum  or 
Articular  Cup  of  the  Pelvis  and  the  Middle  of  the  Head  and  Neck 
of  the  Thigh-bone. 

L.T,  Ligamentum  teres,  or  round  ligament.     The  spaces  marked  with  an  inter- 
rupted line  ( )  represent  the  articular  cartilages.     The  cavity  of  the  synovial 

membrane  is  indicated  by  the  dark  line  between  these,  and,  as  is  shown,  extends  along 
the  neck  of  the  femur  beyond  the  limits  of  the  cartilage.  The  peculiar  shape  of  the 
pelvis  causes  the  section  t3  have  the  remarkable  outline  shown  in  the  cut.  This  will 
be  intelligible  if  compared  with  Fig.  4. 


furnished    by   another.      In    this  case  the   motion   of  the 
former  bone  may  take  place  in  any  direction,  but  the  extent 


348  ELEMENTARY   PHYSIOLOGY  less 

of  the  motion  depends  upon  the  shape  of  the  cup  —  being 
very  great  when  the  cup  is  shallow,  and  small  in  proportion 
as  it  is  deep.  The  shoulder  is  an  example  of  a  ball-and- 
socket  joint  with  a  shallow  cup  (Fig.  5,  B)  ;  the  hip  of  such 
a  joint  with  a  deep  cup  (Fig.  5,  A  and  Fig.  105). 

Hinge-joints  are  single  or  double.  In  the  former  case, 
the  nearly  cylindrical  head  of  one  bone  fits  into  a  corre- 
sponding socket  of  the  other.  In  this  form  of  hinge-joint 
the  only  motion  possible  is  in  the  direction  of  a  plane 
perpendicular  to  the  axis  of  the  cylinder,  just  as  a  door  can 
only  be  made  to  move  round  an  axis  passing  through  its 
hinges.  The  elbow  is  the  best  example  of  this  joint  in  the 
human  body,  but  the  movement  here  is  limited,  because  the 
olecranon,  or  part  of  the  ulna  which  rises  up  behind 
the  humerus,  prevents  the  arm  being  carried  back  behind 
the  straight  line  ;  the  arm  can  thus  be  bent  to,  or  straight- 
ened,.but  not  bent  back  (Fig.  106).  The  knee  (Fig.  104) 
and  ankle  present  less  perfect  specimens  of  a  single  hinge- 
joint. 

A  double  hinge-joint  is  one  in  which  the  articular  surface 
of  each  bone  is  concave  in  one  direction,  and  convex  in 
another,  at  right  angles  to  the  former.  A  man  seated  in  a 
saddle  is  "  articulated  "  with  the  saddle  by  such  a  joint. 
For  the  saddle  is  concave  from  before  backwards,  and  con- 
vex from  side  to  side,  while  the  man  presents  to  it  the 
concavity  of  his  legs  astride,  from  side  to  side,  and  the  con- 
vexity of  his  seat  from  before  backwards. 

The  metacarpal  bone  of  the  thumb  is  articulated  with  the 
bone  of  the  wrist,  called  trapezium,  by  a  double  hinge-joint. 

A  pivot-joint  is  one  in  which  one  bone  furnishes  an  axis, 
or  pivot,  on  which  another  turns  ;  or  itself  turns  on  its  own 
axis,  resting  on  another  bone.  A  remarkable  example  of 
the  former  arrangement  is  afforded  by  the  atlas  and  axis, 


THE  JOINTS   OF  THE   BODY 


349 


or  two  uppermost  vertebras  of  the  neck  (Fig.  107).  The 
axis  possesses  a  vertical  peg,  the  so-called  odontoid  process 
{b),  and  at  the  base  of  the  peg  are  two  obliquely  placed, 
articular  surfaces  («).  The  atlas  is  a  ring-like  bone,  with  a 
massive  thickening  on  each  side.  The  inner  side  of  the 
front  of  the  ring  plays  round  the  neck  of  the  odontoid  peg, 


Fig.  106.  —  Longitudinal  and  Vertical  Section  through  the  Elbow-joint. 

H,  humerus;   £//,ulna;   7V,   the  triceps  muscle,  which   extends  the  arm;  Bi,  the 
biceps  muscle,  which  flexes  it. 


and  the  under  surfaces  of  the  lateral  masses  glide  over  the 
articular  faces  on  each  side  of  the  base  of  the  peg.  A 
strong  ligament  passes  between  the  inner  sides  of  the  two 
lateral  masses  of  the  atlas,  and  keeps  the  hinder  side  of 
the  neck  of  the  odontoid  peg  in  its  place  (Fig.  107,  A). 
By  this  arrangement,  the  atlas  is  enabled  to  rotate  through 


35° 


ELEMENTARY    PHYSIOLOGY 


a  considerable  angle  either  way  upon  the  axis,  without  any 
danger  of  falling  forwards  or  backwards  —  accidents  which 
would  immediately  destroy  life  by  crushing  the  spinal  cord. 
The  lateral  masses  of  the  atlas  have,  on  their  upper  faces, 
concavities  (Fig.  107,  A,  a),  into  which  the  two  convex, 
occipital  condyles  of  the  skull  fit,  and  in  which  they  play 
upwards  and  downwards.  Thus,  the  nodding  of  the  head  is 
effected  by  the  movement  of  the  skull  upon  the  atlas  ;  while, 
in  turning  the  head  from  side  to  side,  the  skull  does  not 
move  upon  the  atlas,  but  the  atlas  slides  round  the  odontoid 
peg  of  the  axis  vertebra. 


Fig.  107. 

A.  The  atlas  viewed  from  above :  a  a,  upper  articular  surfaces  of  its  lateral  masses 
for  the  condyles  of  the  skull ;  b,  the  opening  for  the  peg  of  the  axis  vertebra. 

B.  Side  view  of  the  axis  vertebra;  a,  articular  surface  for  the  lateral  mass  of  the 
atlas;  b,  peg  or  odontoid  process. 


The  second  kind  of  pivot-joint  is  seen  in  the  fore-arm. 

If  the  elbow  and  fore-arm,  as  far  as  the  wrist,  are  made  to 
rest  upon  a  table,  and  the  elbow  is  kept  firmly  fixed,  the 
hand  can  nevertheless  be  freely  rotated  so  that  either  the 
palm,  or  the  back,  is  turned  directly  upwards.  When 
the  palm  is -turned  upwards,  the  attitude  is  called  supination 
(Fig.  108,  A)  ;  when  the  back,  pronation  (Fig.  108,  B). 

The  fore-arm  is  composed  of  two  bones ;  one,  the  ulna, 
which  articulates  with  the  humerus  at  the  elbow  by  the 
hinge-joint  already  described,  in  such  a  manner  that  it  can 


THE  JOINTS   OF  THE   BODY 


351 


move  only  in  flexion  and  extension  (see  p.  348),  and  has 
no  power  of  rotation.  Hence,  when  the  elbow  and  wrist 
are  rested  on  a  table,  this  bone  remains  unmoved. 

But  the  other  bone  of  the  fore-arm,  the  radius,  has  its 
small  upper  end  shaped  like  a  very  shallow  cup  with  thick 
edges.  The  hollow  of  the  cup  articulates  with  a  spheroidal 
surface  furnished  by  the  humerus  :  the  lip  of  the  cup,  with 
a  concave  depression  on  the  side  of  the  ulna. 


Fig.  108. 

The  bones  of  the  right  fore-arm  in  supination  (A)  and  pronation  (B).     H,  humerus; 
R,  radius;    U,  ulna. 


The  large  lower  end  of  the  radius  bears  the  hand,  and 
has,  on  the  side  next  the  ulna,  a  concave  surface,  which 
articulates  with  the  convex  side  of  the  small  lower  end  of 
that  bone. 


352  ELEMENTARY  PHYSIOLOGY  less 

Thus,  the  upper  end  of  the  radius  turns  on  the  double 
surface  furnished  t©  it  by  the  pivot-like  ball  of  the  humerus 
and  the  partial  cup  of  the  ulna ;  while  the  lower  end  of  the 
radius  can  rotate  round  the  surface  furnished  to  it  by  the 
lower  end  of  the  ulna. 

In  supination,  the  radius  lies  parallel  with  the  ulna,  with 
its  lower  end  on  the  outer  side  of  the  ulna  (Fig.  108,  A). 
In  pronation,  it  is  made  to  turn  on  its  own  axis  above,  and 
round  the  ulna  below,  until  its  lower  half  crosses  the  ulna, 
and  its  lower  end  lies  on  the  inner  side  of  the  ulna  (Fig. 
108,  B). 

The  ligaments  which  keep  the  mobile  surfaces  of  bones 
together  are,  in  the  case  of  ball-and-socket  joints,  strong, 
fibrous  capsules,  which  surround  the  joint  on  all  sides.  In 
hinge-joints,  on  the  other  hand,  the  ligamentous  tissue  is 
chiefly  accumulated,  in  the  form  of  lateral  ligaments,  at  the 
sides  of  the  joints.  In  some  cases  ligaments  are  placed 
within  the  joints,  as  in  the  knee,  where  the  bundles  of  fibres 
which  cross  obliquely  between  the  femur  and  the  tibia  are 
called  crucial  ligaments  (Fig.  104,  /)  ;  or,  as  in  the  hip, 
where  the  round  ligament  passes  from  the  bottom  of  the 
socket,  or  acetabulum  of  the  pelvis,  to  the  ball  furnished  by 
the  head  of  the  femur  (Fig.  105,  LT). 

Again,  two  ligaments  pass  from  the  apex  of  the  odontoid 
peg  to  both  sides  of  the  margin  of  the  occipital  foramen, 
i.e.  the  large  hole  in  the  base  of  the  skull,  through  which 
the  spinal  cord  passes  to  join  the  brain;  these,  from  their 
function  in  helping  to  stop  excessive  rotation  of  the  skull, 
are  called  check  ligaments  (Fig.  109,  a). 

In  one  joint  of  the  body,  the  hip,  the  socket  or  aceta- 
bulum (Fig.  105)  fits  so  closely  to  the  head  of  the  femur, 
and  the  capsular  ligament  so  completely  closes  its  cavity 
on  all  sides,  that  the  pressure  of  the  air  must  be  reckoned 


via        THE  VARIOUS    MOVEMENTS   OF  THE   BODY         353 

among  the  causes  which  prevent  dislocation.  This  has 
been  proved  experimentally  by  boring  a  hole  through  the 
floor  of  the  acetabulum,  so  as  to  admit  air  into  its  cavity, 
when  the  thigh-bone  at  once  falls  as  far  as  the  round  and 
capsular  ligaments  will  permit  it  to  do,  showing  that  it  was 
previously  pushed  close  up  by  the  pressure  of  the  external  air. 

13.  The  Various  Movements  of  the  Body.  —  The  differ- 
ent kinds  of  movement  which  the  levers,  thus  connected, 
are  capable  of  performing  are  called  flexion  and  extension  ; 
abduction  and  adduction;  rotation  and  circumduction. 

A  limb  is  flexed,  when  it  is  bent ;  extended,  when  it  is 
straightened  out.  It  is  abducted,  when  it  is  drawn  away 
from  the  middle  line  ;  adducted,  when  it  is  brought  toward 
the  middle  line.  It  is  rotated,  when  it  is  made  to  turn  on  its 
own  axis  5  ch'cumducted,  when  it  is  made  to  describe  a  coni- 
cal surface  by  rotation  round  an  imaginary  axis. 

No  part  of  the  body  is  capable  of  perfect  rotation  like  a 
wheel,  for  the  simple  reason  that  such  motion  would  neces- 
sarily tear  all  the  vessels,  nerves,  muscles,  etc.,  which  unite 
it  with  other  parts. 

Any  two  bones  united  by  a  joint  may  be  moved  one  upon 
another  in,  at  fewest,  two  different  directions.  In  the  case 
of  a  pure  hinge-joint,  these  directions  must  be  opposite  and 
in  the  same  plane  ;  but,  in  all  other  joints,  the  movements 
may  be  in  several  directions  and  in  various  planes. 

In  the  case  of  a  pure  hinge-joint,  the  two  practicable 
movements  —  viz.,  flexion  and  extension  —  may  be  effected 
by  means  of  two  muscles,  one  for  each  movement,  and 
running  from  one  bone  to  the  other,  but  on  opposite  sides 
of  the  joint.  When  either  of  these  muscles  contracts,  it 
will  pull  its  attached  ends  together,  and  bend  or  straighten, 
as  the  case  may  be,  the  joint  towards  the  side  on  which  it 
is  placed.  Thus,  the  biceps  muscle  is  attached,  at  one  end, 
2  A 


354 


ELEMENTARY   PHYSIOLOGY 


to  the  shoulder-blade,  while,  at  the  other  end,  its  tendon 
passes  in  front  of  the  elbow-joint  to  the  radius  (Figs.  98 
and  106,  Bi) :  when  this  muscle  contracts,  therefore,  it 
bends,  or  flexes,  the  fore-arm  on  the  arm.  At  the  back  of 
the  joint  there  is  the  triceps  (Fig.  106,  Tr)  :  when  this 
contracts,  it  straightens,  or  extends,  the  fore-arm  on  the 
arm. 

6-e 


Fig.  iog. 

The  vertebral  column  in  the  upper  part  of  the  neck  seen  from  behind  and  laid  open 
to  show,  a,  the  check  ligaments  of  the  axis;  b,  b' ,  the  broad  ligament  which  extends 
from  the  front  margin  of  the  occipital  foramen  along  the  hinder  faces  of  the  bodies 
of  the  vertebrae;  it  is  cut  through,  and  the  cut  ends  turned  back  to  show,  c,  the  special 
ligament  which  connects  the  point  of  the  odontoid  peg  with  the  front  margin  of  the 
occipital  foramen;  c  is  placed  on  the  occipital  bone;  /,  the  atlas;   //,  the  axis. 


In  the  other  extreme  form  of  articulation — the  ball-and- 
socket  joint  —  movement  in  any  number  of  planes  may  be 
effected,  by  attaching  muscles  in  corresponding  number 
and  direction,  on  the  one  hand,  to  the  bone  which  affords 
the  socket,  and  on  the  other  to  that  which  furnishes  the 
head.  Circumduction  will  be  effected  by  the  combined 
and  successive  contraction  of  these  muscles. 

14.  The  Mechanics  of  Locomotion.  —  We  may  now  pass 
from  the  consideration  of  the  mechanism  of  mere  motion  to 
that  of  locomotion. 


vin  THE   MECHANICS   OF   LOCOMOTION  355 

When  a  man  who  is  standing  erect  on  both  feet  proceeds 
to  walk,  beginning  with  the  right  leg,  the  body  is  inclined, 
so  as  to  throw  the  centre  of  gravity  forward  ;  and,  the  right 
foot  being  raised,  the  right  leg  is  advanced  for  the  length  of 
a  step,  and  the  foot  is  put  down  again.  In  the  meanwhile, 
the  left  heel  is  raised,  but  the  toes  of  the  left  foot  have  not 
left  the  ground  when  the  right  foot  has  reached  it,  so  that 
there  is  no  moment  at  which  both  feet  are  off  the  ground. 
For  an  instant,  the  legs  form  two  sides  of  an  equilateral 
triangle,  and  the  centre  of  the  body  is  consequently  lower 
than  it  was  when  the  legs  were  parallel  and  close  together. 

The  left  foot,  however,  has  not  been  merely  dragged 
away  from  its  first  position,  but  the  muscles  of  the  calf, 
having  come  into  play,  act  upon  the  foot  as  a  lever  of  the 
second  order,  and  thrust  the  body,  the  weight  of  which 
rests  largely  on  the  left  astragalus,  upwards,  forwards,  and 
to  the  right  side.  The  momentum  thus  communicated  to 
the  body  causes  it,  with  the  whole  right  leg,  to  describe  an 
arc  over  the  right  astragalus,  on  which  that  leg  rests  below. 
The  centre  of  the  body  consequently  rises  to  its  former 
height  as  the  right  leg  becomes  vertical,  and  descends  again 
as  the  right  leg,  in  its  turn,  inclines  forward. 

When  the  left  foot  has  left  the  ground,  the  body  is 
supported  on  the  right  leg,  and  is  well  in  advance  of  the 
left  foot ;  so  that,  without  any  further  muscular  exertion, 
the  left  foot  swings  forward  like  a  pendulum,  and  is  carried 
by  its  own  momentum  beyond  the  right  foot,  to  the  position 
in  which  it  completes  the  second  step. 

When  the  intervals  of  the  steps  are  so  timed  that  each 
swinging  leg  comes  forward  into  position  for  a  new  step 
without  any  exertion  on  the  part  of  the  walker,  walking 
is  effected  with  the  greatest  possible  economy  of  force. 
And,  as  the  swinging  leg  is  a  true  pendulum  —  the  time  of 


356  ELEMENTARY   PHYSIOLOGY  less. 

vibration  of  which  depends,  other  things  being  alike,  upon 
its  length  (short  pendulums  vibrating  more  quickly  than 
long  ones),  —  it  follows  that,  on  the  average,  the  natural 
step  of  short-legged  people  is  quicker  than  that  of  long- 
legged  people. 

In  running,  there  is  a  period  when  both  feet  are  off  the 
ground.  The  legs  are  advanced  by  muscular  contraction, 
and  the  lever  action  of  each  foot  is  swift  and  violent. 
Indeed,  the  action  of  each  leg  resembles,  in  violent  running, 
that  which,  when  both  legs  act  together,  constitutes  a  jump, 
the  sudden  extension  of  the  legs  adding  to  the  impetus, 
which,  in  slow  walking,  is  given  only  by  the  feet. 

15.  The  Mechanism  of  the  Larynx.  —  Perhaps  the  most 
singular  motor  apparatus  in  the  body  is  the  larynx,  by  the 
agency  of  which  the  voice  is  produced. 

The  essential  conditions  of  the  production  of  the  human 
voice  are  :  — 

(a)  The  existence  of  the  so-called  vocal  cords. 

(b)  The  parallelism  of  the  edges  of  these  cords,  without 
which  they  will  not  vibrate  in  such  a  manner  as  to  give,  out 
sound. 

((f)  A  certain  degree  of  tightness  of  the  vocal  cords, 
without  which  they  will  not  vibrate  quickly  enough  to 
produce  sound. 

(d)  The  passage  of  a  current  of  air  between  the  parallel 
edges  of  the  vocal  cords  of  sufficient  power  to  set  the  cords 
vibrating. 

The  larynx  (Fig.  no)  is  a  short  tubular  box  opening 
above  into  the  bottom  of  the  pharynx  and  below  into  the 
top  of  the  trachea.  Its  framework  is  supplied  by  certain 
cartilages  more  or  less  movable  on  each  other,  and  these 
are  connected  together  by  joints,  membranes,  and  muscles. 
Across  the  middle  of  the  larynx  is  a  transverse  partition, 


inn 


MECHANISM   OF  THE   LARYNX 


357 


formed  by  two  folds  of  the  lining  mucous  membrane, 
stretching  from  either  side,  but  not  quite  meeting  in  the 
middle  line  (Fig.  in).  They  thus  leave,  in  the  middle 
line,  a  chink  or  slit,  running  from  the  front  to  the  back, 
called  the  glottis.  The  two  edges  of  this  slit  are  not  round 
and  flabby,  but  sharp  and,  so  to  speak,  clean  cut ;  they  are 
also  strengthened  by  a  quantity  of 
elastic  tissue,  the  fibres  of  which  are 
disposed  lengthwise  in  them.  These 
sharp  free  edges  of  the  glottis  are 
the  so-called  vocal  cords,  or  vocal 
ligaments. 

The  thyroid  cartilage  (Fig.  no, 
Tli)  is  a  broad  plate  of  gristle  bent 
upon  itself  into  a  V-shape,  and  so 
disposed  that  the  point  of  the  V  is 
turned  forwards,  and  constitutes  what 
is  commonly  called  "  Adam's  apple." 
Above,  the  thyroid  cartilage  is  at- 
tached by  ligament  and  membrane 
to  the  hyoid  bone  (Fig.  no,  Nv) . 
Below  and  behind,  its  broad  sides  are 
produced  into  little  elongations  or 
horns,  which  are  articulated  by  liga- 
ments with  the  outside  of  a  great  ring 
of  cartilage,  the  cricoid  (Fig.  no, 
Cr),  which  forms,  as  it  were,  the  top 
of  the  windpipe. 

The  cricoid  ring  is  much  higher  behind  than  in  front, 
and  a  gap,  filled  up  by  membrane  only,  is  left  between  its 
upper  edge  and  the  lower  edge  of  the  front  part  of  the 
thyroid,  when  the  latter  is  horizontal.  Consequently,  the 
thyroid  cartilage,  turning  upon  the  articulations  of  its  horns 


Fig.  ho. 

Diagram  of  the  larynx 
seen  from  the  right  side,  the 
thyroid  cartilage  (77/)  being 
supposed  to  be  transparent, 
and  allowing  the  right  ary- 
tenoid cartilage  (Ar),  vocal 
cords  (/"),  and  thyroaryte- 
noid muscle  (ThA),  the 
upper  part  of  the  cricoid  car- 
tilage (CV) ,  and  the  attach- 
ment of  the  epiglottis  (Ep)  to 
be  seen.  C.th,  the  right  crico- 
thyroid muscle:  7V,  the 
trachea;  Hy,  the  hyoid  bone; 
ThA  is  placed  just  below 
the  "Adam's  apple." 


35§ 


ELEMENTARY   PHYSIOLOGY 


with  the  hinder  part  of  the  cricoid,  as  upon  hinges,  can  be 
moved  up  and  down  through  the  space  occupied  by  this 
membrane ;  or,  if  the  thyroid  cartilage  is  fixed,  the  cricoid 
cartilage  moves  in  the  same  way  upon  its  articulations  with 
the  thyroid.  When  the  thyroid  moves  downwards  or  the 
cricoid  upwards,  the  distance  be- 
tween the  front  part  of  the  thyroid 
cartilage  and  the  back  of  the  cri- 
coid is  necessarily  increased  ;  and 
when  the  reverse  movement  takes 
place  the  distance  is  diminished. 
There  is,  on  each  side,  a  large 
muscle,  the  crico-thyroid,  which 
passes  from  the  outer  side  of  the  cri- 
coid cartilage  obliquely  upwards 
and  backwards  to  the  thyroid,  and 
pulls  the  latter  down ;  or,  if  the 
thyroid  is  fixed,  pulls  the  cricoid 
up  (Fig.  no,  ah.). 

Perched  side  by  side  upon  the 
upper  edge  of  the  back  part  of 
the  cricoid  cartilage  are  two  small, 
irregularly  -  shaped  but,  roughly 
speaking,  pyramidal  cartilages,  the 
arytenoid  cartilages  (Figs,  no  and 
1 1 2,  Ary.').  Each  of  these  is  artic- 
ulated by  its  base  with  the  cricoid 
cartilage  by  means  of  a  shallow  joint,  which  permits  of  very 
varied  movements,  and  especially  allows  the  front  portions 
of  the  two  arytenoid  cartilages  to  approach,  or  to  recede 
from,  each  other. 

It  is  to  the  forepart  of  one  of  these  arytenoid  cartilages 
that  the  hinder  end  of  each  of  the  two  vocal  cords  is  fas- 


Fig.       in. — Vertical       and 
Transverse  Section 

through  the  larynx, 
the  Hinder  Half  of  which 
is  removed. 

Ep,  Epiglottis;  Th,  thy- 
roid cartilage;  a,  cavities  called 
the  ventricles  of  the  larynx 
above  the  vocal  cords  (  V) ;  x  the 
right  thyro-arytenoid  muscle  cut 
across;  Cr,  the  cricoid  carti- 
lage. 


THE   MECHANISM   OF  THE    LARYNX 


359 


tened ;  and  they  stretch  from  these  points  horizontally  for- 
ward across  the  cavity  of  the  larynx,  to  be  attached,  close 
together,  in  the  re-entering  angle  of  the  thyroid  cartilage 
rather  lower  than  half-way  between  its  top  and  bottom. 

Now  when  the  arytenoid  cartilages  diverge,  as  they  do 
when  the  larynx  is  in  a  state  of  rest,  it  is  evident  that  the 
aperture  of  the  glottis  will  be  V-shaped,  the  point  of  the  V 
being  forward,  and  the  base  behind  (Figs.  112,  113). 


Fig.   112.  —  The  Parts  surrounding   the  Glottis  partially  dissected    and 
viewed  from  above. 

7V;.,  the  thyroid  cartilage:  Cr.,  the  cricoid  cartilage;  /',  the  edges  of  the  vocal 
cords  bounding  the  glottis;  Ary  ,  the  arytenoid  cartilages;  T/i.A.,  thyro-arytenoid; 
C.a./.,  lateral  crico-arytenoid;  C.a.p.,  posterior  crico-arytenoid;  Ar.p.,  posterior 
arytenoid  muscles. 


For,  in  front,  or  in  the  angle  of  the  thyroid,  the  two  vocal 
cords  are  fastened  permanently  close  together,  whereas, 
behind,  their  extremities  will  be  separated  as  far  as  the 
arytenoids,  to  which  they  are  attached,  are  separated  from 
each  other  (Fig.  113,  I,  B).  Under  these  circumstances 
a  current  of  air  passing  through  the  glottis  produces  no 
sound,  the  parallelism  of  the  vocal  cords  being  wanting ; 


360  ELEMENTARY   PHYSIOLOGY  less. 

whence  it  is  that,  ordinarily,  expiration  and  inspiration  take 
place  quietly.  Passing  from  one  arytenoid  cartilage  to  the 
other,  at  their  posterior  surfaces  are  certain  muscles  called 
the  posterior  arytenoid  (Fig.  112,  Ar.p.).  There  are  also 
two  sets  of  muscles  connecting  each  arytenoid  with  the  cri- 
coid, and  called  from  their  positions  respectively  the  poste- 
rior and  lateral  crico-arytenoid  (Fig.  112,  C.a.p.,  C.aJ.). 
By  the  more  or  less  separate  or  combined  action  of  these 
muscles,  the  arytenoid  cartilages,  and  especially  the  front 
part  of  these  cartilages  and,  consequently,  the  hinder  ends 
of  the  vocal  cords  attached  to  them,  may  be  made  to 
approach  or  recede  from  each  other,  and  thus  the  vocal 
cords  rendered  parallel   (Fig.  113,  I,  A)   or  the  reverse. 

We  have  seen  that  the  crico-thyroid  muscle  pulls  the 
thyroid  cartilage  down,  or  the  cricoid  cartilage  up,  and  thus 
increases  the  distance  between  the  front  of  the  thyroid  and 
the  back  of  the  cricoid,  on  which  the  arytenoids  are  seated. 
This  movement,  the  arytenoids  being  fixed,  must  tend  to 
pull  out  the  vocal  cords  lengthwise,  or,  in  other  words,  to 
tighten  them  (Fig.  114). 

Running  from  the  re-entering  angle  in  the  front  part  of 
the  thyroid,  backwards,  to  the  arytenoids,  alongside  the 
vocal  cords  (and  indeed  imbedded  in  the  transverse  folds, 
of  which  the  cords  are  the  free  edges),  are  two  strong  mus- 
cles, one  on  each  side  (Fig.  112,  T/i.A.),  called  thyro- 
arytenoid. The  effect  of  the  contraction  of  these  muscles 
is  to  pull  up  the  thyroid  cartilage  after  it  has  been  depressed 
by  the  crico-thyroid  muscles  (or  to  pull  down  the  cricoid 
after  it  has  been  raised),  and  consequently  to  slacken  the 
vocal  cords  (Fig.  114). 

Thus,  the  parallelism  {I?)  of  the  vocal  cords  is  determined 
chiefly  by  the  relative  distance  from  each  other  of  the  ary- 
tenoid cartilages ;   the   tension    (c)    of  the  vocal   cords   is 


THE   VOICE 


?6i 


determined  chiefly  by  the  upward  or  downward  movement 
of  the  thyroid  or  cricoid  cartilage ;  and  both  these  con- 
ditions are  dependent  on  the  action  of  certain  muscles. 

The  current  of  air  {d)  whose  passage  sets  the  cords 
vibrating  is  supplied  by  the  movements  of  expiration,  which, 
when  the  cords  are  sufficiently  parallel  and  tense,  produce 
that  musical  note  which  constitutes  the  voice,  but  otherwise 
give  rise  to  no  audible  sound  at  all. 


I  A 


Fig.  113. 

I.  View  of  the  human  larynx  from  above  as  actually  seen  by  the  aid  of  the  instru- 
ment called  the  laryngoscope;  A,  in  the  condition  when  voice  is  being  produced;  B, 
at  rest,  when  no  voice  is  produced. 

ee' ,  epiglottis  (foreshortened). 
c.v,  the  vocal  cords. 

c.v.s,  the  so-called  false  vocal  cords,  folds  of  mucous  membrane  lying  above 
the  real  vocal  cords. 

a,  elevation  caused  by  the  arytenoid  cartilages. 

s,  w,  elevations  caused  by  small  cartilages  connected  with  the  arytenoids. 

/,   root  of  the  tongue. 

II.  Diagram  of  the  same. 


16.  The  Voice. — Voice  consists  simply  of  the  sound,  or 
musical  note,  which  results  from  the  vibration  of  the  vocal 
cords.     Other  things  being  alike,  the  musical   note  will  be 


362  ELEMENTARY  PHYSIOLOGY  less. 

low  or  high,  according  as  the  vocal  cords  are  relaxed  or 
tightened  :  and  this  again  depends  upon  the  relative  pre- 
dominance of  the  contraction  of  the  thyro-arytenoid  and 
crico-thyroid  muscles.  For,  when  the  thyro-arytenoid  mus- 
cles are  fully  contracted,  the  thyroid  cartilage  will  be  raised, 
relatively  to  the  cricoid,  as  far  as  it  can  go,  and  the  vocal 
cords  will  be  rendered  relatively  lax ;  while,  when  the  crico- 
thyroid muscles  are  fully  contracted,  the  thyroid  cartilage 
will  be  depressed,  relatively  to  the  cricoid,  as  much  as  pos- 
sible, and  the  vocal  cords  will  be  made  more  tense. 

If,  while  a  low  note  is  being  sounded,  the  tip  of  the 
finger  be  placed  on  the  crico-thyroid  space  (which  can  be 
felt,  through  the  skin,  beneath  the  lower  edge  of  the  thy- 
roid cartilage),  and  a  high  note  be  then  suddenly  produced, 
the  crico-thyroid  space  will  be  found  to  be  narrowed  by  the 
approximation  of  the  front  edges  of  the  cricoid  and  thyroid 
cartilages.  At  the  same  time,  however,  the  whole  larynx  is, 
to  a  slight  extent,  moved  bodily  upwards  and  thrown  for- 
ward, and  the  cricoid  has  a  particularly  distinct  upward 
movement;  this  movement  of  the  whole  larynx  must  be 
carefully  distinguished  from  the  motion  of  the  thyroid  rela- 
tively to  the  cricoid. 

The  range  of  any  voice  depends  upon  the  difference  of 
tension  which  can  be  given  to  the  vocal  cords,  in  these  two 
positions  of  the  thyroid  cartilage.  Accuracy  of  singing 
depends  upon  the  precision  with  which  the  singer  can  vol- 
untarily adjust  the  contractions  of  the  thyro-arytenoid  and 
crico-thyroid  muscles  —  so  as  to  give  his  vocal  cords  the 
exact  tension  at  which  their  vibration  will  yield  the  notes 
required. 

The  quality  of  a  voice  —  treble,  bass,  tenor,  etc.  —  on 
the  other  hand,  depends  upon  the  make  of  the  particular 
larynx,  the  primitive  length  of  its  vocal  cords,  their  elasticity, 


SPEECH 


363 


the  amount  of  resonance  of  the  surrounding  parts,  and  so 
on. 

Thus,  men  have  deeper  notes  than  boys  and  women, 
because  their  larynxes  are  larger  and  their  vocal  cords 
longer  —  whence,  though  equally  elastic,  they  vibrate  less 
swiftly. 

17.  Speech.  —  Speech  is  voice  modulated  by  the  throat, 
tongue,  and  lips.  Thus,  voice  may  exist  without  speech ; 
and  it   is  commonly  said  that  speech  may  exist  without 


Fig.  114. 

Diagram  of  a  model  illustrating  the  action  of  the  levers  and  muscles  of  the  larynx. 
The  stand  and  vertical  pillar  represent  the  cricoid  and  arytenoid  cartilages,  while  the 
rod  (b  e),  moving  on  a  pivot  at  e.  takes  the  place  of  the  thyroid  cartilage;  a  b  is  an 
elastic  band  representing  the  vocal  cord.  Parallel  with  this  runs  a  cord  fastened  at 
one  end  to  the  rod  b  c,  and,  at  the  other,  passing  over  a  pulley  to  the  weight  B.  This 
represents  the  thyro-arytenoid  muscle.  A  cord  attached  to  the  middle  of  be,  and 
passing  over  a  second  pulley  to  the  weight  A,  represents  the  crico-thyroid  muscle. 
It  is  obvious  that  when  the  bar  {be)  is  pulled  down  to  the  position  ed,  the  elastic 
band  {ab)  is  put  on  the  stretch. 


voice,  as  in  whispering.  This  is  true,  however,  only  if  the 
title  of  voice  be  restricted  to  the  sound  produced  by  the 
vibration  of  the  vocal  cords  ;  for,  in  whispering,  there  is  a 
sort  of  voice  produced  by  the  vibration  of  the  muscular 
walls  of  the  lips,  which  thus  replace  the  vocal  cords.  A 
whisper  is,  in  fact,  a  very  low  whistle. 

The  modulation  of  the  voice  into  speech  is  effected  by 


364  ELEMENTARY   PHYSIOLOGY  less. 

changing  the  form  of  the  cavity  of  the  mouth  and  nose,  by 
the  action  of  the  muscles  which  move  the  walls  of  those 
parts. 

Thus,  if  the  pure  vowel  sounds  — 

E  (as  in  he),  A  (as  in  hay),  A'  (as  in  ah), 

O  (as  in  or),  O'  (as  in  oh),  00  (as  in  cool), 

are  pronounced  successively,  it  will  be  found  that  they  all 
may  be  formed  out  of  the  sound  produced  by  a  continuous 
expiration,  the  mouth  being  kept  open,  but  the  form  of  its 
aperture,  and  the  extent  to  which  the  lips  are  thrust  out  or 
drawn  in  so  as  to  lengthen  or  shorten  the  distance  of  the 
orifice  from  the  larynx,  being  changed  for  each  vowel.  It 
will  be  narrowest,  with  the  lips  most  drawn  back,  in  E,  wid- 
est in  A',  and  roundest,  with  the  lips  most  protruded,  in  00. 

Certain  consonants  also  may  be  pronounced  without  in- 
terrupting the  current  of  expired  air,  by  modification  of  the 
form  of  the  throat  and  mouth. 

Thus  the  aspirate,  H,  is  the  result  of  a  little  extra  expira- 
tory force  —  a  sort  of  incipient  cough.  £  and  Z,  Sh  andy 
(as  in  jugular  =  G  soft,  as  in  gentry),  Th,  L,  R,  E,  V,  may 
likewise  all  be  produced  by  continuous  currents  of  air  forced 
through  the  mouth,  the  shape  of  the  cavity  of  which  is  pecul- 
iarly modified  by  the  tongue  and  lips. 

All  the  vocal  sounds  hitherto  noted  resemble  one  another 
so  far,  that  their  production  does  not  involve  the  stoppage 
of  the  current  of  air  which  traverses  either  of  the  modulat- 
ing passages. 

But  the  sounds  of  J/ and  TV  can  be  formed  only  by  block- 
ing the  current  of  air  which  passes  through  the  mouth,  while 
free  passage  is  left  through  the  nose.  For  M,  the  mouth  is 
shut  by  the  lips;  for  N,  by  the  application  of  the  tongue  to 
the  palate. 


VIII  SPEECH  365 

The  other  consonantal  sounds  of  the  English  language  are 
produced  by  shutting  the  passage  through  both  nose  and 
mouth  ;  and,  as  it  were,  forcing  the  expiratory  vocal  current 
through  the  obstacle  furnished  by  the  latter,  the  character 
of  which  obstacle  gives  each  consonant  its  peculiarity.  Thus, 
in  producing  the  consonants  B  and  P,  the  mouth  is  shut  by 
the  lips,  which  are  then  forced  open  in  this  explosive  man- 
ner. In  T  and  D,  the  mouth  passage  is  suddenly  barred 
by  the  application  of  the  point  of  the  tongue  to  the  teeth, 
or  to  the  front  part  of  the  palate  ;  while  in  K  and  G  (hard, 
as  in  go)  the  middle  and  back  of  the  tongue  are  similarly 
forced  against  the  back  part  of  the  palate. 

An  artificial  larynx  may  be  constructed  by  properly 
adjusting  elastic  bands,  which  take  the  place  of  the  vocal 
cords  ;  and,  when  a  current  of  air  is  forced  through  these, 
due  regulation  of  the  tension  of  the  bands  will  give  rise  to 
all  the  notes  of  the  human  voice.  As  each  vowel  and  con- 
sonantal sound  is  produced  by  the  modification  of  the  length 
and  form  of  the  cavities  which  lie  over  the  natural  larynx, 
so,  by  placing  over  the  artificial  larynx  chambers  to  which 
any  requisite  shape  can  be  given,  the  various  letters  may  be 
sounded.  It  is  by  attending  to  these  facts  and  principles 
that  various  speaking  machines  have  been  constructed. 

Although  the  tongue  is  credited  with  the  responsibility  of 
speech,  as  the  "  unruly  member,"  and  undoubtedly  takes  a 
very  important  share  in  its  production,  it  is  not  absolutely 
indispensable.  Hence,  the  apparently  fabulous  stories  of 
people  who  have  been  enabled  to  speak  after  their  tongues 
had  been  cut  out  by  the  cruelty  of  a  tyrant,  or  persecutor, 
may  be  quite  true. 

Some  years  ago  I  had  the  opportunity  of  examining  a 
person,  whom  I  will  call  Mr.  R.,  whose  tongue  had  been 
removed  as  completely  as  a  skilful  surgeon  could  perform 


366  ELEMENTARY   PHYSIOLOGY  less,  viii 

the  operation.  When  the  mouth  was  widely  opened,  the 
truncated  face  of  the  stump  of  the  tongue,  apparently  cov- 
ered with  new  mucous  membrane,  was  to  be  seen,  occupying 
a  position  as  far  back  as  the  level  of  the  anterior  pillars  of 
the  fauces.  The  dorsum  of  the  tongue  was  visible  with  diffi- 
culty ;  but  I  believe  I  could  discern  some  of  the  circumval- 
late  papillae  upon  it.  None  of  these  were  visible  upon  the 
amputated  part  of  the  tongue,  which  had  been  preserved  in 
spirit ;  and  which,  so  far  as  I  could  judge,  was  about  2\ 
inches  long. 

When  his  mouth  was  open,  Mr.  R.  could  advance  his 
tongue  no  further  than  the  position  in  which  I  saw  it ;  but 
he  informed  me  that  when  his  mouth  was  shut  the  stump 
of  the  tongue  could  be  brought  much  more  forward. 

Mr.  R.'s  conversation  was  perfectly  intelligible  ;  and  such 
words  as  think,  the,  cow,  kill,  were  well  and  clearly  pro- 
nounced. But  tin  became  fin;  tack,  fack  or  pack ;  toll, 
pool;  dog,  thog;  dine,  vine;  dew,  thew ;  cat,  cat/;  mad, 
mad/;  goose,  gooth  ;  big,  pig,  bich,  pick,  with  a  guttural  ch. 

In  fact,  only  the  pronunciation  of  those  letters  the  forma- 
tion of  which  requires  the  use  of  the  tongue  was  affected ; 
and,  of  these,  only  the  two  which  involve  the  employment 
of  its  tip  were  absolutely  beyond  Mr.  R.'s  power.  He  con- 
verted all  t's  and  d's  into/V,  p's,  v's,  or  th's.  Th  was  fairly 
given  in  all  cases  ;  s  and  sh,  I  and  r,  with  more  or  less  of  a 
lisp.  Initial  g*s  and  k's  were  good  ;  but  final  g's  were  all 
more  or  less  guttural.  In  the  former  case,  the  imperfect 
stoppage  of  the  current  of  air  by  the  root  of  the  tongue  was 
of  no  moment,  as  the  sound  ran  on  into  that  of  the  follow- 
ing vowel ;  while,  when  the  letter  was  terminal,  the  defect 
at  once  became  apparent. 


LESSON    IX 

SENSATIONS   AND    SENSORY    ORGANS 

1.  Movement  the  Result  of  Reflex  Action.  —  The  agent 
by  which  all  the  motor  organs  (except  the  cilia)  described 
in  the  preceding  Lesson  are  set  at  work,  is  muscular  fibre. 
But,  in  the  living  body,  muscular  fibre  is,  as  a  rule,  made  to 
contract  by  a  change  which  takes  place  in  the  motor  or  effe- 
rent nerve  which  is  distributed  to  it.  This  change  again  is 
generally  effected  by  the  activity  of  the  central  nervous  sys- 
tem, with  which  the  motor  nerve  is  connected.  The  central 
organ  is  thrown  into  activity,  directly  or  indirectly,  by  the 
influence  of  changes  which  take  place  in  nerves,  called  sen- 
sory or  afferent,1  which  are  connected,  on  the  one  hand,  with 
the  central  organ,  and,  on  the  other  hand,  with  some  other 
part,  usually  on  the  surface,  of  the  body.  Finally,  the  altera- 
tion of  the  afferent  nerve  is  itself  produced  by  changes  in 
the  condition  of  the  part  of  the  body  with  which  it  is  con- 
nected ;  which  changes  usually  result  from  external  impres- 
sions brought  to  bear  on  that  part. 

Sometimes  the  central  organ  enters  into  a  state  of  activity 
without  our  being  able  to  trace  that  activity  to  any  direct 
influence  of  changes  in  afferent  nerves ;  the  activity  seems 
to  take  origin  in  the  central  organ,  and  the  movements  to 
which  it  gives  rise  are  called  "  spontaneous,"  or  "  voluntary." 
Putting  these  cases  on  one  side,  it  may  be  stated  that  a 

1  It  should  be  mentioned  that  not  all  efferent  nerves  are  motor,  nor  all 
afferent  nerves  sensory.     Compare  p.  500. 

367 


3b8  ELEMENTARY   THYSIOLOGY  less. 

movement  of  the  body,  or  of  a  part  of  it,  is  to  be  regarded 
as  the  effect  of  an  influence  (technically  termed  a  stimulus) 
applied  directly,  or  indirectly,  to  the  ends  of  afferent  netves, 
and  giving  rise  to  a  modification  of  the  condition  of  the 
particles  or  molecules  which  form  the  substance  of  the  nerve 
fibres,  i.e.,  to  a  molecular  change  called  a  nervous  impulse, 
which  is  propagated  from  molecule  to  molecule  along  the 
fibres  to  the  central  nervous  system  with  which  these  are 
connected.  The  molecular  activity  of  the  afferent  nerve 
sets  up  changes  of  a  like  order  in  the  fibres  and  cells  of  the 
centra]  organ ;  from  these  the  disturbance  is  transmitted 
along  the  motor  nerves,  which  pass  from  the  central  organ 
to  certain  muscles.  And,  when  the  disturbance  in  the  molec- 
ular condition  of  the  efferent  nerves  reaches  the  endings  of 
those  nerves  in  muscular  fibres,  a  similar  disturbance  is  com- 
municated to  the  substance  of  the  muscular  fibres,  whereby, 
in  addition  to  the  production  of  certain  other  phenomena,  to 
which  reference  has  already  been  made  (p.  320),  the  parti- 
cles of  the  muscular  substance  are  made  to  take  up  a  new 
position,  so  that  each  fibre  shortens  and  becomes  thicker, 
and  a  movement  ensues.  Thus,  for  instance,  if  we  uninten- 
tionally prick  one  of  our  fingers  or  touch  some  very  hot 
object  the  hand  is  jerked  away  almost  before  we  are  aware 
of  what  has  happened. 

Such  a  series  of  molecular  changes  as  that  just  described  is 
called  a  reflex  action  :  the  disturbance  in  the  afferent  nerves 
caused  by  the  irritation  being  as  it  were  reflected  back,  along 
the  efferent  nerves,  to  the  muscles.  But  the  name  is  not  a 
good  one,  since  it  seems  to  imply  that  the  molecular  changes 
in  the  afferent  nerve,  the  central  organ,  and  the  efferent  nerve 
are  all  alike,  and  differ  only  in  direction ;  whereas  there  is 
reason  to  think  that  they  differ  in  many  ways. 

The  several  structures  necessary  for  the  carrying  out  of  a 


SENSATIONS   AND   CONSCIOUSNESS 


369 


muscular  contraction,  resulting  in  movement,  in  the  way  we 
have  described,  may  be  made  clear  by  the  following  diagram 
(Fig.  115). 

The  stimulus  is  applied  to  a  sensory  surface  (S)  ;  the 
change  thus  set  up  is  propagated  as  a  nervous  impulse  along 
the  sensory  (afferent)  nerve  a.f.  to  c,  a  part  of  the  central 
nervous  system  (the  spinal  cord).  The  changes  which 
then  take  place  in  c.  result  in  the  setting  up  of  a  nervous 


Fig.  115.  —  Diagram  to  illustrate  the  Paths  of  Reflex  Action. 

Sp.C.  spinal  cord.  S,  some  sensory  surface;  a.f.  afferent  or  sensory  nerve;  c. 
central  connection  in  nervous  system;  e.f.,  e.f.  efferent  or  motor  nerves;  M1,  M2, 
muscles.     The  arrows  show  the  directions  in  which  the  impulses  travel. 


impulse  in  the  motor  (efferent)  nerve  e.f.,  which  is  conveyed 
outwards  along  that  nerve  to  the  muscle  M1,  usually  on  the 
same  side  of  the  body.  Sometimes  the  impulse  is  sent  out 
along  a  motor  nerve  to  some  muscle,  M2,  on  the  opposite 
side  of  the  body. 

2.  Sensations  and  Consciousness.  —  A  reflex  action  may 
take  place  without  our  knowing  anything  about  it,  and  hun- 
dreds of  such  actions  are  continually  going  on  in  our  bodies 
without  our  being  aware  of  them.  But  it  very  frequently 
happens  that  we  learn  that  something  is  going  on,  when  a 

2B 


370  ELEMENTARY   PHYSIOLOGY  less. 

stimulus  affects  our  afferent  nerves,  by  having  what  we  call 
a  feeling  or  sensation.  We  class  sensations  along  with  emo- 
tions and  volitions  and  thoughts,  under  the  common  head 
of  states  of  consciousness.  But  what  consciousness  is  we 
know  not ;  and  how  it  is  that  anything  so  remarkable  as  a 
state  of  consciousness  comes  about  as  the  result  of  irritating 
nervous  tissue  is  just  as  unaccountable  as  any  other  ultimate 
fact  of  nature. 

Sensations  are  of  very  various  degrees  of  definiteness. 
Some  arise  within  ourselves,  we  know  not  how  or  where, 
and  remain  vague  and  undefinable.  Such  are  the  sensations 
of  uncomfortableness,  of  faintness,  of  fatigue,  or  of  restless- 
ness. We  cannot  assign  any  particular  place  to  these  sensa- 
tions, which  are  very  probably  the  result  of  affections  of  the 
afferent  nerves  in  general,  brought  about  by  the  state  of  the 
blood,  or  that  of  the  tissues  in  which  they  are  distributed. 
However  real  these  sensations  may  be,  and  however  largely 
they  enter  into  the  sum  of  our  pleasures  and  pains,  they  tell 
us  absolutely  nothing  of  the  external  world.  They  are  not 
only  diffuse,  but  they  are  also  subjective  sensations. 

3.  The  Special  Senses.  —  In  the  case  of  other  sensations, 
each  feeling  arises  out  of  changes  taking  place  in  a  definite 
part  of  the  body,  is  produced  by  a  stimulus  applied  to  that 
part  of  the  body,  and  cannot  be  produced  by  stimuli  applied 
to  other  parts  of  the  body.  Thus,  the  sensations  of  taste 
and  smell  are  confined  to  certain  regions  of  the  mucous 
membrane  of  the  mouth  and  nasal  cavities  ;  those  of  sight 
and  hearing  to  the  particular,  parts  of  the  body  called  the 
eye  and  the  ear ;  and  those  of  touch,  though  arising  over  a 
much  wider  area  than  the  others,  are  nevertheless  restricted 
to  the  skin  and  to  some  portions  of  the  membranes  lining  the 
internal  cavities  of  the  body.  Any  portion  of  the  body  to 
which  a  sensation  is  thus  restricted  is  called  a  sense-organ. 


ex  THE   GENERAL   PLAN   OF   A    SENSE-ORGAN         371 

It  may  be  here  remarked  that,  in  the  case  of  the  sensation 
of  touch,  the  simple  feeling  of  contact  is  accompanied  by 
information,  not  only  as  to  what  sense-organ,  but  also  as  to 
what  part  of  that  sense-organ,  is  being  affected.  When  we 
touch  a  hot  or  a  rough  body  with  the  tip  of  a  finger,  we 
are  aware  not  only  that  we  are  dealing  with  a  hot  or  a 
rough  body,  but  also  that  the  hot  or  rough  body  is  in  con- 
tact with  the  tip  of  the  finger ;  we  "  refer,"  as  is  said,  the 
sensation  to  that  part  of  the  tip  of  the  finger  which  is  being 
acted  upon  by  the  body  in  question.  With  the  other  sensa- 
tions the  case  is  different.  When  we  smell  a  bad  smell, 
though  we  know  that  we  smell  by  the  nose,  we  do  not  con- 
sider that  the  smell  arises  in  the  nose  ;  we  conclude  that 
there  is  some  object  outside  ourselves  which  is  causing  the 
bad  smell.  We  refer  the  origin  of  the  sensation  to  some 
external  cause,  and  that  even  when  the  sensation  is  after  all 
due  to  changes  taking  place  in  the  nose  itself  independently 
of  external  objects,  as  in  the  unpleasant  odours  which  accom- 
pany certain  diseases  of  the  nose.  Similarly,  all  our  sensa- 
tions of  sight  and  of  hearing  are  referred  to  external  objects  ; 
and  even  in  the  case  of  taste,  when  a  lump  of  sugar  is  taken 
into  the  mouth,  we  are  simply  aware  of  a  sensation  of  sweet- 
ness and  do  not  associate  that  sensation  of  sweetness  with 
any  particular  part  of  the  mouth,  though,  by  the  sense  of 
touch,  which  the  inside  of  the  mouth  also  possesses,  we  can 
tell  pretty  exactly  whereabouts  in  the  mouth  the  melting  lump 
is  lying. 

4.  The  General  Plan  of  a  Sense-organ.  —  In  these  sensa- 
tions, thus  arising  in  special  sense-organs,  and  hence  often 
spoken  of  as  "special"  sensations,  each  sensation  or  feeling 
results  from  the  application  of  a  particular  kind  of  stimulus 
to  its  appropriate  sense-organ ;  and,  in  each  case,  the  struc- 
ture of  the  sense-organ  is  arranged  in  such  a  manner  as  to 


372  ELEMENTARY   PHYSIOLOGY  less. 

render  that  organ  peculiarly  sensitive  to  its  appropriate 
stimulus. 

Thus,  the  sensations  of  sight  are  brought  about  by  the 
action  of  the  vibrations  of  the  luminiferous  ether  ;  and  the 
eye,  or  sense-organ  of  sight,  is  constructed  in  such  a  way 
that  rays  of  light,  which  falling  on  any  other  part  of  the 
body  produce  no  appreciable  effect,  give  rise  to  vivid  sensa- 
tions when  they  fall  upon  it. 

Further,  we  may,  with  more  or  less  completeness,  distin- 
guish in  each  sense-organ  two  parts :  an  essential  part, 
through  which  the  agent  producing  the  sensation  (be  it 
light,  a  series  of  sonorous  vibrations,  a  sapid  or  odorous 
chemical  substance,  a  change  in  temperature,  or  a  variation 
in  pressure)  produces  changes  in  certain  structures  which 
are  peculiarly  associated  with  the  delicate  terminations  of 
the  nerve  distributed  to  the  sense-organ ;  and  an  accessory- 
part,  not  absolutely  necessary  to  the  sense,  but  of  great  use- 
fulness inasmuch  as  it  assists  in  bringing  the  agent  to  bear, 
in  the  most  efficient  way,  upon  the  essential  part.  In  the 
case  of  the  eye  and  ear  this  accessory  part  is  extremely 
complicated,  and,  indeed,  seems  to  form  the  greater  part  of 
the  whole  sense-organ  ;  in  the  case  of  the  other  senses  4  is 
much  more  simple. 

The  essential  part  of  each  sense-organ  is  in  turn  composed 
of  minute  organs,  which  upon  examination  appear  to  be  in 
reality  modified  epithelial  cells  ;  and  the  delicate  termina- 
tions of  the  nerve  filaments  distributed  to  the  sense-organ 
may,  with  more  or  less  distinctness,  be  traced  to  the  immedi- 
ate vicinity  of  these  modified  cells.  These  minute  organs, 
these  modified  epithelial  cells,  may  be  spoken  of  as  sense- 
organules  ;  they  serve  as  intermediators  in  each  case  between 
the  physical  agent  of  the  sensation  and  the  sensory  nerve. 
The  physical   agent  is  by  itself  unable  to  produce  in  the 


ix  THE   SKIN   AS  A   SENSE-ORGAN  373 

fibres  of  the  sensory  nerve  those  changes  which,  reaching 
the  brain  as  nervous  impulses,  give  rise  to  the  special  sensa- 
tions. Thus,  as  we  shall  presently  see,  rays  of  light  falling 
upon  the  optic  nerve  cannot  give  rise  to  a  sensation  of  sight. 
The  physical  agent  must  act  first  on  the  sense-organules,  and 
these  in  turn  act  upon  the  filaments  of  the  nerve.  Thus, 
light,  falling  upon  the  sense-organules  situated  in  that  essen- 
tial part  of  the  eye  called  the  retina,  sets  up  changes  in  them, 
these  changes  set  up  corresponding  changes  in  the  delicate 
nerve  filaments  which  with  the  sense-organules  go  to  make 
up  the  retina,  and  the  changes  in  the  nerve  filaments  propa- 
gated along  the  optic  nerve  to  the  brain  give  rise,  in  the 
latter,  to  sensations  of  sight. 

Hence  in  the  essential  part  of  each  sense-organ  we  have 
to  distinguish  between  the  sense-organules,  i.e.  the  modified 
epithelium,  and  the  terminal  expansion  of  the  sensory  nerve  ; 
and  further,  in  each  sense-organ,  there  is  added  to  this  essen- 
tial part  a  more  or  less  complicated  accessory  part. 

Lastly,  in  all  these  special  sensations,  there  are  certain 
phenomena  which  arise  out  of  the  structure  of  the  sense- 
organ,  and  others  which  result  from  the  operation  of  the 
central  apparatus  of  the  nervous  system  upon  the  materials 
supplied  to  it  by  the  sense-organ. 

5.  The  Skin  as  a  Sense-Organ. —  The  sense  of  touch 
(including  the  senses  of  pressure,  temperature,  and  pain)  is 
possessed,  more  or  less  acutely,  by  all  parts  of  the  free  sur- 
face of  the  body,  and  by  the  walls  of  the  mouth  and  nasal 
passages. 

Whatever  part  possesses  this  sense  consists  of  a  membrane 
(integumentary  or  mucous)  composed  of  a  deep  layer  made 
up  of  fibrous  tissue  containing  a  capillary  network,  and  of  a 
superficial  layer  consisting  of  epidermal  or  epithelial  cells, 
among  which  are  no  vessels.      (Sec  p.  215.) 


374 


ELEMENTARY   PHYSIOLOGY 


Wherever  the  sense  of  touch  is  delicate,  the  deep  layer  is 
not  a  mere  flat  expansion,  but  is  raised  up  into  multitudes 
of  small,  close-set,  conical  elevations  (see  Fig.  65,  p.  216), 
which  are  called  papillae.  In  the  skin,  the  coat  of  epithelial 
or  epidermal  cells  does  not  follow  the  contour  of  these  pa- 
pillae, but  dips  down  between  them  and  forms  a  tolerably 

even  coat  over  them.  Thus, 
the  points  of  the  papillae  are 
much  nearer  the  surface  than 
the  general  plane  of  the  deep 
layer  whence  these  papillae 
proceed.  Loops  of  vessels  en- 
ter the  papillae,  and  sensory 
nerve-fibres  are  distributed  to 
them.  In  some  cases  the 
nerve-fibre  ends  in  a  papilla 
in  a  definite  organ,  in  what  is 
called  a  tactile  corpuscle,  or 
in  a  similar  body  called  an 
end-bulb.  Each  of  these  or- 
gans consists  essentially  of  an 
oval  or  rounded  swelling, 
formed  by  a  modification  and 
enlargement  of  the  delicate 
connective  tissue  ensheathing 
the  nerve-fibre  ;  in  the  mid- 
dle of  the  swelling  the  nerve-fibre  itself  ends  abruptly  in  a 
peculiar  manner.  These  bodies  are  especially  found  in  the 
papillae  of  those  localities  which  are  endowed  with  a  very 
delicate  sense  of  touch,  as  in  the  tips  of  the  fingers,  the 
point  of  the  tongue,  etc. ;  and  the  papillae  which  contain 
tactile  corpuscles  generally  contain  few  or  no  blood-vessels. 
Tactile  corpuscles  (Fig.  116)  occur  most  numerously  in 


Fig.  116.  —  Tactile  Corpuscle  within 
a  Papilla  of  the  Skin  of  the 
Hand.     (Ranvier.) 

«,  «,  two  nerve-fibres  passing  to  the 
corpuscle;  a,  a,  varicose  terminations 
of  the  nerve-fibres  inside  the  corpuscle. 


END-BULBS 


375 


the  papillae  of  the  skin  of  the  palmar  surface  of  the  hand, 
especially  of  the  finger  tips  ;  they  are  also  present,  but  much 
less  numerously,  on  the  plantar  surfaces  of  the  skin  of  the 
feet,  and  are  commonest  on  parts  of  the  skin  where  there  is 
no  hair.  Each  corpuscle  forms  an  elongated,  bulbous  swell- 
ing about  75/*  (3^0  inch)  in  length  at  the  end  of  the  nerve- 
fibre  to  which  it  is  attached,  and  lies  with  its  long  axis  in  the 
long  axis  of  the  papilla  (t.c,  Fig.  65,  p.  216).  The  corpuscle 
consists  of  a  sheath  or  capsule  of  connective  tissue  which 
sends  into  the  interior  incomplete  transverse  partitions.  The 
nerve  which  supplies  the  cor- 
puscle approaches  it  at  its  side, 
winds  once  or  twice  around  it, 
then  enters  the  body  of  the  cor- 
puscle, and  divides  into  a  number 
of  branches,  which  end  in  enlarge- 
ments. 

End-bulbs  (Fig.  117)  are  found 
in  the  papilla?  of  the  skin  of  the   Fig.  117  -  End-bulb  from  the 

,.  t  •  1  •  •  m.  Human  Conjunctiva.1   (Long- 

lips  and  in  other  situations,    ihey       worth.) 

are    Spheroidal   and    Smaller   (/tOu        .  <*,   the    nerve-fibre;    b.    capsule 

with  nuclei;  c,  c,  portions  of  nerve- 
in  diameter)  than  the  tactile  COr-     fibre  inside  the  end-bulb;  d,  e,  cells 

of  the  core. 

puscles.    They  are  not  all  exactly 

alike,  but  the  commonest  form  consists  of  a  thin  outer 
sheath  or  capsule,  which  is  nucleated  and  incloses  a  mass  of 
polygonal  cells.  The  nerve-fibre  enters  the  capsule  and 
ends  among  the  cells  in  its  interior. 

The  great  majority,  however,  of  the  nerve-fibres  going  to 
the  skin  do  not  end  in  any  such  definite  organs.  They  divide 
in  the  dermis  into  exceedingly  delicate  minute  filaments, 
the   course  and  ultimate  terminations  of  which  are  traced 


1  The  conjunctiva  is  the  mucous  membrane  which  lines  the  eyelids  and 
covers  the  front  of  the  eyeball. 


376  ELEMENTARY  PHYSIOLOGY  less, 

with  the  greatest  difficulty.  Some  of  the  finest  filaments, 
however,  pass  into  the  epidermis  and  are  there  lost  among,  or 
possibly  connected  with,  some  of  the  epidermal  cells,  espe- 
cially those  of  the  lower  layers. 

Another  kind  of  highly  specialised  nerve-ending  is  found 
on  the  branches  of  the  nerves  which  supply  the  skin  of  the 
hand  and  foot,  as  they  pass  through  the  subcutaneous  tissue, 
and  in  other  places.  These  are  known  as  Pacinian  corpus- 
cles, called  after  Pacini,  an  Italian  anatomist  born  in  1812, 
who  first  carefully  described  them.  From  their  position  they 
are  not,  strictly  speaking,  sensory  endings  of  nerves  in  the 
skin ;  but  they  possess  undoubtedly  some  sensory  functions, 
although  we  do  not  know  what  these  may  be. 

The  Pacinian  corpuscles  (Fig.  118)  are  long,  ovoid,  bul- 
bous structures  of  considerable  size,  averaging  ^  of  an  inch 
in  length.  They  are  thus  easily  visible  to  the  naked  eye. 
Each  corpuscle  consists  of  an  elaborate  capsule  containing 
an  elongated  central  core  of  homogeneous  material  in  which 
the  axis  of  the  nerve  is  imbedded  and  terminates.  The 
capsule  consists  of  some  30  to  40  capsules,  made  of  connec- 
tive tissue,  and  placed  one  outside  the  other  like  the  layers 
of  an  ordinary  onion. 

It  is  obvious,  from  what  has  been  said,  that  no  direct  con- 
tact takes  place  between  a  body  which  is  touched  and  the 
sensory  nerve,  —  a  thicker  or  thinner  layer  of  epithelium,  or 
epidermis,  being  situated  between  the  two.  In  fact,  if  this 
layer  is  removed,  as  when  a  surface  of  the  skin  has  been 
blistered,  contact  with  the  raw  surface  gives  rise  to  a  sense 
of  pain,  not  to  one  of  touch  properly  so  called.  Thus,  in 
touch,  the  essential  part  of  the  sense-organ  consists  either 
of  certain  epithelial  or  epidermal  cells  of  the  general  in- 
tegument or  of  certain  structures  contained  in  the  tactile 
corpuscles,  end-bulbs,  and  other  similar  organs.    These  epi- 


PACINIAN   CORPUSCLES 


377 


thelial  cells,  very  slightly  modified  apparently  in  the  general 
skin,  but  more  so  in  the  tactile  corpuscles  and  end-bulbs, 
are  the  sense-organules ;  they  serve  as  intermediators  be- 
tween the  physical  agent  —  pressure  —  and  the  terminal 
filaments  of  the  sensory  nerves.     The  accessory  part  of  the 


Fig.  118.  —  A  Pacinian  Corpuscle  from  a  Cat's  Mesentery.     (Ranvier.) 
n,  nerve-fibre,  passing  through  the  core,  m.,  and  terminating  at  a. 

sense-organ  of  touch  is  very  slightly  developed,  being  chiefly 
supplied  by  the  variable  number  and  form  of  the  papillae 
and  the  variable  thickness  and  character  of  the  layers  of 
epidermal  cells. 


378  ELEMENTARY   PHYSIOLOGY  less. 

(i)  The  Sensation  of  Pressure.  —  Mere  contact  of  a  single 
object  with  the  skin  exerts  a  pressure  on  it  which  results  in 
a  stimulation  by  means  of  which  we  become  aware  that 
something  is  touching  us.  The  power  of  discriminating 
pressure  and  its  differences  we  may  call  the  sense  of  press- 
ure. The  sensitiveness  of  the  various  regions  of  the  skin 
in  responding  to  pressure  varies,  and  the  difference  may  be 
measured  for  each  part  of  the  skin  by  determining  either 
what  the  least  weight  is  which  can  be  just  felt  when  allowed 
to  rest  on  that  part,  or  else  by  determining  the  least  differ- 
ence in  weight  which  can  be  distinguished  between  two 
weights  laid  in  succession  on  the  same  spot.  Experiment- 
ing in  this  way  it  may  be  shown  that  the  sense  of  pressure 
is  most  acute  on  the  skin  of  the  forehead  and  of  the  back 
of  the  hand.  The  sense  is  less  acute  in  the  skin  of  the 
finger  tips.  Careful  investigation  seems  to  show,  with  but 
little  doubt,  that  some  points  on  the  skin  of  any  part  are 
peculiarly  sensitive  to  pressure.  They  are  spoken  of  as 
"  pressure  spots."  They  are  believed  to  overlie  the  end- 
ings of  the  nerves  which  mediate  the  sensations  of  pressure, 
but  what  the  end-organs  of  the  sense  are  is  not  known. 

(ii)  The  Sensations  of  Temperature. — The  feeling  of 
warmth,  or  cold,  is  the  result  of  an  excitation  of  sensory 
nerves  distributed  to  the  skin,  which  are  possibly  distinct 
from  those  which  give  rise  to  the  sense  of  pressure.  And 
it  would  appear  that  the  heat  must  be  transmitted  through 
the  epidermal  or  epithelial  layer  to  give  rise  to  this  sensa- 
tion ;  for,  just  as  touching  a  naked  nerve,  or  the  trunk  of  a 
nerve,  gives  rise  only  to  pain,  so  heating  or  cooling  an  ex- 
posed nerve,  or  the  trunk  of  a  nerve,  gives  rise  not  to  a 
sensation  of  heat  or  cold,  but  simply  to  pain.  Thus,  if  the 
elbow  be  dipped  into  a  mixture  of  ice  and  salt,  the  cold  first 
affects  the  skin  of  the  elbow,  giving  rise  to  a  sensation  of 


THE   SENSATIONS   OF  TEMPERATURE 


379 


cold  at  the  elbow,  but  afterwards  attacks  the  trunk  of  the 
ulnar  nerve,  which  at  the  elbow  lies  not  very  far  below  the 
skin ;  and  this  latter  effect  is  felt  as  a  sensation,  not  of  cold 
but  of  pain.  The  pain,  moreover,  thus  caused  is  not  felt 
in  the  trunk  of  the  nerve  at  the  elbow,  where  the  cold  is 
acting,  but  in  the  parts  where  the  fibres  of  the  nerve  end, 
more  particularly  in  the  little  and  ring  fingers. 


Fig.  119.  — Outlines  of  Heat  Spots  and  Cold  Spots.     (After  Goldscheider.) 

The  heat  spots  are  cross-hatched  and  dark,  the  cold  spots  are  dotted  and  light.     In 
some  places  the  heat  spots  and  cold  spots  overlap  each  other. 


Again,  the  sensation  of  heat,  or  cold,  is  relative  rather 
than  absolute.  Suppose  three  basins  be  prepared,  one  filled 
with  ice-cold  water,  one  with  water  as  hot  as  can  be  borne, 
and  the  third  with  a  mixture  of  the  two.  If  the  hand  be 
put  into  the  hot-water  basin,  and  then  transferred  to  the 
mixture,  the  latter  will  feel  cold  ;  but  if  the  hand  be  kept  a 


380  ELEMENTARY   PHYSIOLOGY  less. 

while  in  the  ice-cold  water,  and  then  transferred  to  the  very 
same  mixture,  this  will  feel  warm. 

Like  the  sense  of  pressure,  the  sense  of  warmth  varies  in 
delicacy  in  different  parts  of  the  body.  The  cheeks  are 
very  sensitive,  more  so  than  the  lips ;  the  palms  of  the 
hands  are  more  sensitive  to  heat  than  their  backs.  Hence 
a  washerwoman  holds  her  flat-iron  to  her  cheek  to  test  the 
temperature,  and  one  who  is  cold  spreads  the  palms  of  his 
hands  to  the  fire. 

The  differences  in  the  sensitiveness  of  the  skin  to  heat 
and  cold  at  various  points  may  be  readily  determined  by 
touching  the  several  points  with  the  blunt  end  of  a  wire 
whose  temperature  can  be  kept  constant  at  any  desired 
degree.  In  this  way  it  is  found  that  some  points  respond 
to  heat  but  not  to  cold,  others  to  cold  but  not  to  heat,  so 
that  we  meet  with  "heat  spots"  and  "cold  spots."  The 
accompanying  figure  (Fig.  119)  shows  the  distribution  of 
these  spots  in  a  small  area  of  the  skin  of  the  thigh.  Their 
localisation  is  different  from  that  of  the  "  pressure  spots." 
They  probably  mark  the  position  of  the  terminal  organs  of 
heat  and  cold,  but  these,  like  the  organs  of  pressure,  are 
unknown. 

(iii)  The  Sensation  of  Pain.  —  Pain  is  often  regarded  as 
the  result  of  an -excessive  stimulation  of  any  of  the  nerve- 
endings  which  are  concerned  in  giving  rise  to  sensations. 
Pain  also  results  from  stimulating  the  trunks  of  the  nerves 
leading  from  those  endings  to  the  central  nervous  system. 
In  the  latter  case  the  pain  is  "  referred  "  outwards  to  the 
end  of  the  nerve,  as  in  the  experiment  of  cooling  the  elbow 
described  above.  The  nerves  of  any  part  may  thus  give 
rise  to  pain,  from  this  it  might  appear  that  we  can  scarcely 
speak  of  any  distinct  and  separate  "  sense  "  of  pain.  But 
there  are  certain  facts  which  show  that  sensations  of  pain 


ix  LOCALISATION  OF  TACTILE   SENSATIONS  381 

arc  probably  distinct  from,  though  ultimately  mixed  up 
with,  other  sensations.  Thus,  in  many  diseases  of  the  ner- 
vous system,  such  as  locomotor  ataxy,  the  sensitiveness  of 
the  skin  to  touch  may  be  almost  entirely  wanting,  while  pain 
is  readily  felt.  Further,  observation  shows  that  the  impulses 
giving  rise  to  pain,  as  also  those  resulting  from  heat  and 
cold,  pass  along  the  spinal  cord  on  their  way  to  the  brain 
by  paths  which  are  distinct  from  those  which  convey  the 
impulses  resulting  from  mere  touch  or  pressure. 

(iv)  The  Localisation  of  Tactile  Sensations.  —  Certain  very 
curious  phenomena  appertain  to  the  sense  of  touch ;  some 
of  these  are  probably  in  part  due  to  varying  anatomical 
arrangements,  to  the  varying  thickness  of  the  epidermis, 
and  to  the  abundance  or  scantiness  of  special  end-organs. 
Not  only  is  tactile  sensibility  to  a  single  impression  much 
duller  in  some  parts  than  in  others  —  a  circumstance  which 
might  in  many  cases  be  accounted  for  by  the  different  thick- 
ness of  the  epidermal  layer  —  but  the  power  of  distinguish- 
ing double  simultaneous  impressions  is  very  different.  Thus, 
if  the  ends  of  a  pair  of  compasses  (which  should  be  blunted 
with  pointed  pieces  of  cork)  are  separated  by  only  one-tenth 
or  one-twelfth  of  an  inch,  they  will  be  distinctly  felt  as  two,  if 
applied  to  the  tips  of  the  fingers  ;  whereas,  if  applied  to  the 
back  of  the  hand  in  the  same  way,  only  one  impression  will 
be  felt ;  and,  on  the  arm,  they  may  be  separated  for  a  quarter 
of  an  inch,  and  still  only  one  impression  will  be  perceived. 

Accurate  experiments  have  been  made  in  different  parts 
of  the  body,  and  it  has  been  found  that  two  points  can  be 
distinguished  by  the  tongue,  if  only  one  twenty-fourth  of  an 
inch  apart ;  by  the  tips  of  the  fingers  if  one  twelfth  of  an 
inch  distant ;  while  they  may  be  one  inch  distant  on  the 
cheek  or  forehead,  and  even  three  inches  on  the  back,  and 
still  give  rise  to  only  one  sensation. 


382  ELEMENTARY   PHYSIOLOGY  less. 

6.  The  Muscular  Sense.  —  What  is  termed  the  muscular 
sense  is  less  vaguely  localised  than  the  sensations  referred 
to  above  in  Section  2  (p.  370),  though  its  place  is  still 
incapable  of  being  very  accurately  defined.  This  muscular 
sensation  is  largely  the  feeling  of  resistance  which  arises 
when  any  kind  of  obstacle  is  opposed  to  the  movement  of 
the  body,  or  of  any  part  of  it ;  and  it  is  something  quite 
different  from  the  feeling  of  contact  or  even  of  pressure. 

Lay  one  hand  fiat  on  its  back  upon  a  table,  and  rest  a 
disc  of  cardboard  a  couple  of  inches  in  diameter  upon  the 
ends  of  the  outstretched  fingers ;  the  only  result  will  be  a 
sensation  of  contact —  the  pressure  of  so  light  a  body  being 
inappreciable.  But  put  a  two-pound  weight  upon  the  card- 
board, and  the  sensation  of  contact  will  pass  into  what 
appears  to  be  a  very  different  feeling,  viz.,  that  of  pressure. 
Up  to  this  moment  the  fingers  and  arm  have  rested  upon 
the  table  ;  but  now  let  the  hand  be  raised  from  the  table, 
and  another  new  feeling  will  make  its  appearance  —  that  of 
resistance  to  effort.  This  feeling  comes  into  existence  with 
the  exertion  of  the  muscles  which  raise  the  arm  ;  and  it  is 
the  consciousness  of  that  exertion  which  goes  by  the  name 
of  "  the  muscular  sense." 

Any  one  who  raises  or  carries  a  weight  knows  well  enough 
that  he  has  this  sensation  :  but  he  may  be  greatly  puzzled  to 
say  where  he  has  it.  Nevertheless,  the  sense  itself  is  very 
delicate,  and  enables  us  to  form  tolerably  accurate  judg- 
ments of  the  relative  intensity  of  resistances.  Persons  who 
deal  in  articles  sold  by  weight  are  constantly  enabled  to  form 
very  precise  estimates  of  the  weight  of  such  articles  by  bal- 
ancing them  in  their  hands  ;  and  in  this  case  they  depend 
in  a  great  measure  upon  the  muscular  sense. 

But  the  muscular  sense  embraces  more  than  the  mere  con- 
sciousness of  the   resistance  to  effort  involved  in  lifting  a 


ix  THE   SENSE    OF  TASTE  383 

weight.  Thus,  it  is  a  matter  within  everybody's  experience 
that,  even  when  the  eyes  are  closed,  we  are  perfectly  well 
aware  of  the  direction  and  extent  of  any  movement  of  any 
part  of  the  body.  Moreover  we  are  equally  conscious  of  the 
position  of  any  part  of  the  body  at  any  moment,  whether  the 
position  is  the  result  of  our  own  voluntary  movement  or 
the  result  of  the  action  of  some  other  person,  who  has  placed 
the  part  in  position.  In  all  such  cases  the  muscular  sense 
supplies  the  basis  of  our  knowledge  of  the  position  or  of  the 
movements  of  the  parts  of  our  body. 

The  muscular  sense  is  thus  essentially  concerned  with  sen- 
sations arising  from  movements,  whether  active  or  passive. 
Now  the  parts  affected  by  these  movements  are  chiefly  the 
following  four ;  the  skin,  the  muscles,  the  tendons,  and  the 
ligaments.  It  has  been  supposed  that  the  impulses  which 
give  rise  to  the  sensations  may  be  largely  due  to  the  stimu- 
lation of  cutaneous  nerves  resulting  from  the  varying  extent 
to  which  the  skin  is  put  on  the  stretch  by  the  movements ; 
but  the  arguments  in  favour  of  this  view  are  not  conclusive. 
On  the  other  hand,  we  know  that  the  muscles  themselves  and 
the  ligaments  at  the  joints  possess  nerve-fibres  which  are  cer- 
tainly afferent,  i.e.,  sensory  ;  and  similarly  afferent  fibres,  con- 
nected with  extremely  minute  end-bulbs,  are  distributed  to 
the  tendons.  And  there  is  but  little  doubt  that  we  must  look 
to  the  impulses  generated  in  these  nerves  as  providing  the 
sensations  which  form  the  basis  of  the  muscular  sense. 

7.  The  Sense  of  Taste.  —  The  organ  of  the  sense  of  taste 
is  the  mucous  membrane  which  covers  the  tongue,  especially 
its  back  part,  and  the  hinder  part  of  the  palate.  Like  that 
of  the  skin,  the  deep,  or  vascular,  layer  of  the  mucous  mem- 
brane of  the  tongue  is  raised  up  into  papillae  (Fig.  120)  ; 
but  these  are  large,  separate,  and  have  separate  coats  of  epi- 
thelium.    Towards  the  tip  of  the  tongue  they  are  for  the 


3»4 


ELEMENTARY    PHYSIOLOGY 


most  part  elongated  and  pointed,  and  are  called  filiform ; 
over  the  rest  of  the  surface  of  the  tongue  these  are  mixed 
with  larger  papillae,  with  broad  ends  and  narrow  bases,  called 
fungiform  {F.p.)  ;  but  towards  its  root  there  are  a  number 
of  still  larger  papillae,  arranged  in  the  figure  of  a  V  with  its 
point  backwards,  each  of  which  is  like  a  fungiform  papilla 


Fig.  120.  —  The  Mouth  widely  opened  to  show  the  Tongue  anp  Palate. 

Uv,  the  uvula;  Th,  the  tonsil  between  the  anterior  and  posterior  pillars  of  the 
fauces;  C.p,  circumvallate  papillae;  F.p,  fungiform  papilla;.  The  minute  filiform 
papillae  cover  the  interspaces  between  these.  On  the  right  side  the  tissues  are 
partially  dissected  to  show  the  course  of  the  filaments  of  the  trigeminal  nerve,  V, 
and  the  glossopharyngeal  nerve,  VIII. 

surrounded  by  a  wall.     These  are  the  circumvallate  papillae 
(Fig.  1 20,  C.p,  and  121,  A). 

In  both  the  fungiform  and  circumvallate  papillae,  the  cells 
which  are  specially  concerned  in  giving  rise  to  sensations  of 


TASTE-BUDS 


J8S 


taste  are  arranged  in  bulbous  groups,  somewhat  like  the 
leaves  in  a  bud,  and  hence  these  groups  are  known  as  taste- 
buds.  In  the  circumvallate  papillae  these  taste-buds  lie  im- 
bedded in  the  layers  of  epithelium  which  cover  the  sides  of 
each  papilla. 

Each  "  bud  "  (Fig.  121,  B)  is  flask-shaped  and  consists  of 
an  outer  wall,  made  up  of  elongated  cells  placed  side  by  side 
like  the  staves  of  a  barrel  (vr)  and  leaving  an  opening  at  the 
end  of  the  bud  where  it  comes  to  the  surface  of  the  papilla. 


Fig.  121. — Diagram  of  a  Circumvallate  Papilla,  and  of  Taste-buds. 

A.  A  circumvallate  papilla  cut  lengthwise;  e,  epidermis;  d,  dermis;  t,  taste- 
buds;   n,  nerve-fibres. 

B.  Two  taste-buds;  e,  epidermis;  d,  dermis;  c,  the  outer  or  cover  cells  shown  in 
the  lower  bud;  n,  four  inner  or  gustatory  cells  with  processes;  in,  processes  project- 
ing at  mouth  of  buds. 

The  inside  of  the  bud  is  filled  with  the  gustatory  cells  (;/), 
packed  side  by  side.  Each  of  these  cells  is  long  and  very 
thin,  with  a  large  nucleus  at  its  middle  point,  and  each  cell 
has  at  its  outer  end  a  delicate  process,  like  a  stiff  cilium  (but 
not  vibratile),  which  projects  through  the  open  mouth  of  the 
bud. 

The  papillae  are  very  vascular,  and  they  receive  nervous 
filaments  from  two  sources,  the  one  the  nerve  called  glosso- 
pharyngeal, the  other  the  gustatory,  which  is  a  branch  0! 


386  ELEMENTARY  PHYSIOLOGY  less. 

the  fifth  nerve  (p.  537).  The  latter  chiefly  supplies  the 
front  and  sides  of  the  tongue,  the  former  its  back  and  the 
adjacent  part  of  the  palate  ;  and  there  is  reason  to  believe 
that  different  taste  sensations  are  supplied  by  the  two 
nerves. 

The  peculiar  cells  in  the  taste-buds  are  the  sense-organ- 
ules  of  taste,  and  constitute  the  essential  part  of  the  organ  of 
taste.  The  nerve-fibres  enter  the  taste-buds  and  terminate 
amongst  the  gustatory  cello.  The  tongue  itself,  which  by  its 
movements  brings  the  sapid  substances  into  immediate  con- 
tact with  these  modified  epithelium  cells,  may  be  regarded 
as  the  accessory  part  of  the  organ  of  taste. 

The  great  majority  of  the  sensations  we  call  taste,  how- 
ever, are  in  reality  complex  sensations,  into  which  smell,  and 
even  touch,  and  the  temperature  sense,  as  in  the  sensation 
of  cold  produced  by  peppermint,  largely  enter.  When  the 
sense  of  smell  is  interfered  with,  as  when  the  nose  is  held 
tightly  pinched,  it  is  very  difficult  to  distinguish  the  tastes  of 
various  objects.  A  piece  of  onion,  for  instance,  the  eyes 
being  shut,  may  then  easily  be  confounded  with  a  bit  of 
apple.  This  explains  the  not  uncommon  device  of  pinching 
the  nose  when  taking  nauseous  medicine. 

But  the  so-called  "  tastes,"  which  are  thus  affected  by  the 
absence  of  smell,  ought  rather  to  be  spoken  of  as  "  flavours  " 
than  as  tastes.  They  are  distinctly  due  to  the  odoriferous 
particles  the  substances  emit,  and  thus  people  are  in  the 
habit  of  "  sniffing  "  a  glass  of  wine  in  order  to  appreciate 
what  they  call  its  taste.  True  taste  is  independent  of  smell, 
as  in  the  case  of  sugar  or  quinine.  When  we  come  to  investi- 
gate the  matter  closely,  we  find  that  the  various  real  tastes 
may  be  arranged  under  four  heads  :  these  are  —  sweet,  bitter, 
sour  or  acid,  and  salt.  These  tastes  are  not  excited  equally 
all  over  the  surface  of  the  tongue.     Thus,  the  tip  is  most 


IX  THE   SENSE  OF   SMELL  38? 

sensitive  to  sweet  and  salt  substances,  and  the  back  to  bitter, 
while  the  sides  of  the  tongue  most  readily  respond  to 
acids. 

The  sense  of  taste  is  most  acute  at  the  temperature  of  the 
body,  and  substances  to  be  tasted  must  be  in  solution. 

8.  The  Sense  of  Smell.  —  The  organ  of  the  sense  of  smell 
is  the  delicate  mucous  membrane  which  lines  the  upper  part 
of  the  nasal  cavities.  In  this  part  the  mucous  membrane 
is  distinguished  from  the  rest  of  the  mucous  membrane  of 
these  cavities  —  first,  by-  the  character  of  its  cells  and  by 
possessing  no  cilia ;  secondly,  by  receiving  a  large  nervous 
supply  from  the  olfactory,  or  first,  pair  of  cerebral  nerves 
(p.  535),  as  well  as  a  certain  number  of  filaments  of  the 
fifth  pair,  whereas  the  rest  of  the  mucous  membrane  is  sup- 
plied from  the  fifth  pair  alone. 

Each  nostril  leads  into  a  spacious  nasal  chamber,  sepa- 
rated, in  the  middle  line,  from  its  fellow  of  the  other  side, 
by  a  partition,  or  septum,  formed  partly  by  cartilage  and 
partly  by  bone,  and  continuous  with  that  partition  which 
separates  the  two  nostrils  one  from  the  other.  Below,  each 
nasal  chamber  is  separated  from  the  cavity  of  the  mouth 
by  a  floor,  the  bony  palate  (Figs.  122  and  123)  ;  and  when 
this  bony  palate  comes  to  an  end,  the  partition  is  continued 
down  to  the  root  of  the  tongue  by  a  fleshy  curtain,  the  soft 
palate,  which  has  been  already  described.  The  soft  palate 
and  the  root  of  the  tongue  together  constitute,  under  ordi- 
nary circumstances,  a  movable  partition  between  the  mouth 
and  the  pharynx ;  and  it  will  be  observed  that  the  opening 
of  the  larynx,  the  glottis,  lies  behind  the  partition  :  so  that 
when  the  root  of  the  tongue  is  applied  close  to  the  soft 
palate  no  passage  of  air  can  take  place  between  the  mouth 
and  the  pharynx.  But  in  the  upper  part  of  the  pharynx 
above  the  partition  are  the  two  hinder  openings  of  the  nasal 


?88 


ELEMENTARY    PHYSIOLOGY 


cavities  (which  are  called  the  posterior  nares)  separated  by 
the   termination   of  the   septum  ;    and   through   these  wide 


Fig.  122. — Vertical  Longitudinal  Sections  of  the  Nasal  Cavity. 

The  upper  figure  represents  the  outer  wall  of  the  left  nasal  cavity;  the  lower 
figure  the  right  side  of  the  middle  partition,  or  septum  (Sfi.)  of  the  nose,  which  forms 
the  inner  wall  of  the  right  nasal  cavity.  /,  the  olfactory  nerve  and  its  branches;  I", 
branches  of  the  fifth  nerve;  Pa,  the  palate,  which  separates  the  nasal  cavity  from 
that  of  the  mouth;  S.T,  the  superior  turbinal  bone;  M.  T,  the  middle  turbinal;  I.T, 
the  inferior  turbinal.  The  letter  /is  placed  in  the  cerebral  cavity;  and  the  partition 
on  which  the  olfactory  lobe  rests,  and  through  which  the  filaments  of  the  olfactory 
nerves  pass,  is  the  cribriform  plate.  In  the  upper  figure  the  branches  of  the  olfac- 
tory nerve  are  represented  as  coming  somewhat  too  far  down. 


IX  THE   SENSE   OF   SMELL  389 

openings  the  air  passes,  with  great  readiness,  from  the 
nostrils  along  the  lower  part  of  each  nasal  chamber  to  the 
glottis,  or  in  the  opposite  direction.  It  is  by  means  of 
the  passages  thus  freely  open  to  the  air  that  we  breathe,  as 
we  ordinarily  do,  with  the  mouth  shut. 

Each  nasal  chamber  rises,  as  a  high  vault,  far  above  the 
level  of  the  arch  of  the  posterior  nares  —  in  fact,  about  as 
high  as  the  depression  of  the  root  of  the  nose.  The  upper- 
most and  front  part  of  its  roof,  between  the  eyes,  is  formed 
by  a  delicate  horizontal  plate  of  bone,  perforated  like  a  sieve 
by  a  great  many  small  holes,  and  thence  called  the  cribri- 
form plate  (Fig.  123,  Cr.).  It  is  this  plate  alone  (with  the 
membranous  structures  which  line  its  two  faces)  which,  in 
this  region,  separates  the  cavity  of  the  nose  from  that  which 
contains  the  brain.  The  olfactory  lobes,  which  are  directly 
connected  with,  and  form  indeed  a  part  of,  the  brain, 
enlarge  at  their  ends,  and  their  broad  extremities  rest  upon 
the  upper  side  of  the  cribriform  plate,  sending  through  it 
immense  numbers  of  delicate  filaments,  the  olfactory  nerves, 
which  are  distributed  as  follows  (Fig.  122)  :  — 

On  each  wall  of  the  septum  the  mucous  membrane  forms 
a  flat  expansion,  but  on  the  side  walls  of  each  nasal  cavity  it 
follows  the  elevations  and  depressions  of  the  inner  surfaces 
of  what  are  called  the  upper  and  middle  turbiiial  or  spongy 
bones.  These  bones  are  called  spongy  because  the  interior 
of  each  is  occupied  by  air  cavities  separated  from  each  other 
by  very  delicate  partitions  only,  and  communicating  with 
the  nasal  cavities.  Hence  the  bones,  though  massive-look- 
ing, are  really  exceedingly  light  and  delicate,  and  fully 
deserve  the  appellation  of  spongy  (Fig.   123). 

Over  the  upper  turbiiial  bones,  and  on  both  sides  of  the 
septum  opposite  to  them,  the  mucous  membrane  is  specially 
modified,  and  receives  the  name  of  olfactory  mucous  mem- 


390 


ELEMENTARY   PHYSIOLOGY 


brane ;  and  it  is  to  this  olfactory  mucous  membrane  that  the 
filaments  of  the  olfactory  nerve  passing  through  the  cribri- 
form plate  are  distributed. 

There  is  a  third  light  scroll-like  bone  distinct  from  these 
two,  and  attached  to  the  maxillary  bone,  which  is  called  the 
inferior  turbinal,  as  it  lies  lower  than  the  other  two,  and  im- 
perfectly separates  the  air  passages  from  the  proper  olfactory 
chamber  (Figs.  122,  123).  It  is  covered  by  the  ordinary 
ciliated  mucous  membrane  of  the  nasal  passage,  and  receives 
no  filaments  from  the  olfactory  nerve. 


.An. 


Fig.  123.  —  A  Transverse  and  Vertical  Section  of  the  Osseous  Walls  of 
the  Nasal  Cavity  taken  nearly  through  the  letter  /  in  the  Fore- 
going Figure. 

Cr,  the  cribriform  plate;  S.T,  M .  T,  the  chambered  superior  and  middle  turbinal 
bones  on  the  former  of  which  and  on  the  septum  (5/.)  the  filaments  of  the  olfactory 
nerve  are  distributed;  /.  T,  the  inferior  turbinal  bone;  PL  the  palate;  An.  the  antrum 
or  chamber  which  occupies  the  greater  part  of  the  maxillary  bone  and  opens  into  the 
nasal  cavity. 


In  the  non-olfactory  part  of  the  nasal  mucous  membrane 
the  epithelium  cells  are  ordinary  ciliated  epithelium  cells 
(see  p.  308),  and  many  glands  secreting  mucus  are  present ; 
but  in  the  olfactory  part  the  epithelium  cells  not  only  lose 
their  cilia,  but  become  peculiarly  modified. 


IX 


THE   SENSE   OF   SMELL 


391 


They  are  of  two  kinds  and  somewhat  similar  to  the  cells 
composing  a  taste-bud  ;  but  their  arrangement  is  different,  the 
two  kinds  being  intermingled.  One  kind  of  cell  is  long, 
slender  and  rod-shaped,  with  a  large  nucleus  towards  its  inner 
end  (Fig.  124,  b).  Those  of  the  second  kind  are  also  thin 
and  rod-like  at  their  inner  ends,  but 
beyond  the  nucleus  the  outer  end 
is  wide  and  columnar  (a).  The 
cells  of  the  first  kind,  which  are  the 
more  numerous,  are  supposed  to 
be  specially  concerned  in  giving 
rise  to  the  sensations  of  smell. 
The  delicate  olfactory  nerve  fila- 
ments appear  to  end  in  these 
modified  epithelial  cells,  which,  in- 
deed, are  the  sense-organules  of 
the  organ  of  smell.  The  olfactory 
mucous  membrane  thus  constitutes 
the  essential  part  of  the  organ. 

The  accessory  part  of  the  organ 
of  smell  may  be  described  as 
follows  :  — 

From  the  arrangements  which 
have  been  described,  it  is  clear 
that,  under  ordinary  circumstances, 
the  gentle  inspiratory  and  expira- 
tory currents  will  flow  along  the 
comparatively  wide,  direct  passages 

afforded    by    SO    much    of   the    nasal     filament  from  the  olfactory  nerve. 

chamber  as  lies  below  the  middle  turbinal ;  and  that  they 
will  hardly  move  the  air  inclosed  in  the  narrow  interspace 
between  the  septum  and  the  upper  and  middle  spongj 
bones,  which  is  the  proper  olfactory  chamber. 


Fig.  124.  —Cells of  Olfactory 
Epithelium.     (Max  Schcltze.) 

1,  From  a  frog;   2,  from  man. 

a,  columnar  epithelial  cell; 
b,  olfactory  rod-cell;  c,  outer 
limb,  ti,  inner  limb  of  olfactory 
cell,  the  former  being  prolonged 
at  e  into  fine  hairs,  the  latter 
being   continuous   with    a    nerve 


392  ELEMENTARY   PHYSIOLOGY  less 

If  the  air  currents  are  laden  with  particles  of  odorous 
matter,  these  can  only  reach  the  olfactory  membrane  by 
diffusing  themselves  into  this  narrow  interspace ;  and,  if 
there  be  but  few  of  these  particles,  they  will  run  the  risk 
of  not  reaching  the  olfactory  mucous  membrane  at  all,  unless 
the  air  in  contact  with  it  be  exchanged  for  some  of  the  odor- 
iferous air.  Hence  it  is  that,  when  we  wish  to  perceive  a 
faint  odour  more  distinctly,  we  "  sniff"  or  snuff  up  the  air. 
Each  sniff  is  a  sudden  inspiration,  the  effect  of  which  must 
reach  the  air  in  the  olfactory  chamber  at  the  same  time  as, 
or  even  before,  it  affects  that  at  the  nostrils  ;  and  thus  must 
tend  to  draw  a  little  air  out  of  that  chamber  from  behind. 
At  the  same  time,  or  immediately  afterwards,  the  air  sucked 
in  at  the  nostrils  entering  with  a  sudden  vertical  rush,  part 
of  it  must  tend  to  flow  directly  into  the  olfactory  chamber, 
and  replace  that  thus  drawn  out. 

The  loss  of  smell  which  takes  place  in  the  course  of  a 
severe  cold  may,  in  part,  be  due  to  the  swollen  state  of  the 
mucous  membrane  which  covers  the  inferior  turbinal  bones, 
impeding  the  passage  of  odoriferous  air  to  the  olfactory 
chamber. 

Very  little  is  known  of  the  physiology  of  smell,  and  smells 
have  not  so  far  been  classified  except  as  agreeable  or  the 
reverse  ;  but  recent  observations  seem  to  show  that  a  much 
more  detailed  classification  is  possible.  Everyday  experi- 
ence shows  that  the  sense  is  extremely  delicate,  the  most 
minute  amount  of  odoriferous  matter,  such  as  musk,  serving 
to  excite  it.  The  sense  is,  however,  much  more  highly  de- 
veloped in,  and  much  more  important  in  the  daily  lives  of, 
some  of  the  lower  animals,  such  as  the  dog,  than  in  man. 

9.  The  Ear  and  the  Sense  of  Hearing  in  General.  —  The 
ear,  or  organ  of  the  sense  of  hearing,  is  very  much  more 
complex  than  any  of  the  sensory  organs  yet  described ;  and 


rv  THE   EAR   AND   THE   SENSE   OF   HEARING  393 

in  it  the  accessory  parts  especially  are  much  more  highly 
developed. 

The  essential  part,  on  each  side  of  the  head,  lies  in  the 
walls  of  a  very  peculiarly  formed  membranous  bag.  This 
bag,  when  the  ear  first  begins  to  be  formed,  is  a  simple 
round  sac,  but  it  subsequently  takes  on  a  very  complicated- 
form,  and  becomes  divided  into  several  parts,  which  receive 
special  names.  It  is  lodged  in  a  cavity  of  correspondingly 
intricate  shape,  hollowed  out  of  a  solid  mass  of  bone  (called 
from  its  hardness  petrous) ,  which  forms  part  of  the  temporal 
bone,  and  lies  at  the  base  of  the  skull.  The  sac,  however, 
does  not  completely  fill  the  cavity,  so  that  a  space  is  left 
between  the  bony  walls  and  the  contained  sac.  This  space, 
which  is  continuous  all  round  the  sac,  being  interrupted  at 
certain  places  only  where  the  membranous  sac  is  attached 
to  the  bony  walls,  contains  a  fluid  provided  by  the  lym- 
phatics of  the  neighbourhood,  and  called  perilymph. 

The  membranous  sac,  the  walls  of  which  consist  chiefly 
of  connective  tissue,  is  lined  by  an  epithelium,  and  contains 
a  fluid  of  its  own  called  endolymph.  The  perilymph,  it 
will  be  understood,  is  quite  distinct  from  the  endolymph, 
the  two  fluids  being  separated  by  the  walls  of  the  membra- 
nous sac. 

Over  a  great  part  of  the  interior  of  the  membranous  sac 
the  epithelium  is  simple  in  character,  but  at  certain  places 
to  be  presently  described  it  assumes  special  features,  being 
greatly  thickened,  and  bearing  hairlike  processes,  or  being 
otherwise  modified,  so  as  to  be  easily  affected  by  even  such 
slight  movements  as  the  vibrations  which  produce  sound. 
Where  these  patches  or  tracts  of  modified  or  auditory  epi- 
thelium, as  it  is  called,  exist,  the  membranous  sac  is  more 
closely  attached  to  the  bony  walls  ;  and  branches  of  the 
eighth,  acoustic  or  auditory,  nerve    (see  p.  537),  passing 


394  ELEMENTARY   PHYSIOLOGY  less. 

through  channels  in  the  bony  walls,  through  the  tissue 
attaching  the  membranous  sac  to  the  bony  walls,  and 
through  the  wall  of  the  membranous  sac  itself,  come  into 
peculiar  relation  with,  and  end  among,  the  cells  of  these 
patches  of  auditory  epithelium.  It  is  only  to  the  places 
where  the  epithelium  is  thus  modified  that  filaments  of  the 
auditory  nerve  are  distributed.  The  auditory  epithelium 
constitutes  the  essential  part  of  the  sense-organ  of  hearing. 

The  membranous  sac  is  known  as  the  membranous  laby- 
rinth, and  the  bony  cavity  in  which  it  lies  is  similarly  called 
the  osseous  labyrinth ;  together  they  constitute  the  internal 
ear.  Outside  of  this  lies  the  middle  ear,  or  drum,  and  still 
further  outward  is  the  external  passage  opening  upon  the 
side  of  the  head,  which  with  the  pinna,  or  "  ear  "  in  popular 
language,  constitutes  the  external  ear.  All  of  these  parts 
except  the  auditory  epithelium  are  accessory  parts  of  the 
organ  of  hearing. 

What  takes  place  in  hearing  may  briefly  be  stated  as  fol- 
lows. The  vibrations  set  up  by  a  sounding  body  are  con- 
ducted, by  the  accessory  apparatus  to  be  presently  described, 
to  the  perilymph,  and  from  thence  through  the  walls  of  the 
membranous  sac  to  the  endolymph.  As  the  vibrations  trav- 
elling along  the  endolymph  reach  those  particular  places 
where  the  epithelium  is  modified,  and  where  the  filaments 
of  the  auditory  nerve  end,  they  in  some  way  or  other  affect 
the  epithelium  cells.  Through  the  intermediation  of  these 
cells  the  delicate  endings  of  the  auditory  nerve  are  stimu- 
lated, so  that  molecular  changes  constituting  a  nervous 
impulse  are  set  up  in  the  substance  of  the  nerve,  and  trans- 
mitted along  the  nerve  from  particle  to  particle,  until  they 
reach  that  part  of  the  brain  the  molecular  disturbance  of 
which  gives  rise  to  sensations  of  sound. 

Thus,  until  the  auditory  epithelium  is  reached,  that  which 


THE   MEMBRANOUS    LABYRINTH 


395 


takes  place  in  the  ear  when  we  hear  a  sound  is  simply  a 
transmission  of  vibrations  of  the  same  order  as  those  which 
are  produced  by  the  sounding  body ;  but  the  processes 
which  intervene  between  the  epithelium  and  the  brain  are 
not  of  the  same  kind  ;  here  there  is  no  transmission  of  such 
vibrations,  but  what  takes  place  is  a  series  of  changes  of 
nerve  substance  of  the  same  order  as,  though  perhaps  not 
exactly  like,  those  which  are  set  up  by  the  action  of  a  stimu- 
lus on  any  other  nerve. 

10.  The  Membranous  Labyrinth.  —  The  membranous 
bag,  as  we  have  said,  is  not  simple  but  complicated  :  it 
consists  of  several  parts,  namely,  the  utricle,  the  saccule, 
the  membranous  semicircular  canals,  and  the  membranous 
cochlea. 

(i)  The  Utricle,  the  Saccule,  and  the  Membranous  Semicircu- 
lar Canals.  —  The  utricle  is  a  somewhat  ovoid  sac  (Fig.  125, 
IT),  into  which  open  the  three  hooplike,  semicircular  canals. 
Of  these,  two  are  placed  vertically  :  one  is  situated  high  up 
and  directed  anteriorly  and  outwards,  the  other  is  lower  and 
directed  posteriorly  and  outwards;  they  are  called  the  supe- 
rior (A.S.C)  and  posterior  (P.S.C)  semicircular  canals. 
The  third  is  placed  horizontally  and  directed  outwards, 
hence  it  is  called  the  external  or  horizontal  semicircular 
canal  (Fig.  125,  E.S.C).  The  three  canals  thus  lie  nearly 
at  right  angles  to  one  another  in  the  three  directions  of 
space  ;  this  has  nothing  to  do  with  judging  the  directions  of 
sound,  but  has  a  relation  to  other  functions  of  the  canals. 
Each  of  these  three  hoops  is  dilated  at  one  of  its  two  ends, 
where  it  opens  into  the  utricle,  into  what  is  called  an  ampulla 
(Fig.  125),  the  other  end  having  no  ampulla.  Thus  there 
is  one  ampulla  to  each  canal.  Those  ends  of  the  two  verti- 
cal canals  which  are  not  dilated  into  ampullae  join  together 
before  they  open  into  the  utricle. 


59b 


ELEMENTARY   PHYSIOLOGY 


LESS, 


In  each  ampulla  is  a  ridge  or  crest,  called  crista  acustica, 
placed  crosswise,  and  projecting  into  the  cavity  of  the  canal. 
Each  crest  is  formed  partly  by  an  infolding  and  thickening 
of  the  connective  tissue  wall  of  the  ampulla,  and  partly  by  a 
thickening  of  the  epithelium,  which  here  has  the  peculiar 
characters  already  referred  to.  A  similar  but  oval  patch  of 
thickened,  modified,  auditory  epithelium,  with  a  thickening 
of  the  wall  beneath  it,  is  found  in  the  utricle  itself;  this  is 
called  a  macula  acustica. 


E.S.C 


AJF 


BS.C 


Coch 


Fig.   125.  —  Diagram    to   illustrate   the    Membranous  Labyrinth  and   the 
endings  of  the  auditory  nerve. 

U,  utricle,  containing  a  macula  acustica;  A.S.C,  E  S.C,  P.S.C,  superior,  exter- 
nal, and  posterior  semicircular  canals;  in  each  case  the  letters  point  to  the  ampulla  of 
the  canal,  which  contains  a  crista  acustica;  S,  saccule,  containing  a  macula  acustica  ; 
A.  V.,  canal  uniting  the  utricle  with  the  saccule;  Coch,  cochlea,  with  the  nerve  fila- 
ments supplying  the  organ  of  Corti;  c,  canal  uniting  the  saccule  with  the  cochlea; 
A.N,  auditory  nerve  dividing  into  several  branches. 

Attached  to  the  utricle  is  a  similar  smaller  sac  (forming 
another  division  of  the  primitive  membranous  bag)  called 
the  saccule  (Fig.  125,  s),  on  the  walls  of  which  is  a  similar 
rounded  patch  of  modified  epithelium,  or  macula  acustica. 
The  cavity  of  the  saccule  is  cut  off  from  that  of  the  utricle, 
except  for  a  curious  roundabout  connection  by  means  of  a 
narrow  canal  (Fig-  125,  A.V.). 


ix  THE   MEMBRANOUS   LABYRINTH  397 

Branches  of  the  auditory  nerve  pass  to  these  parts  of  the 
membranous  labyrinth  and  send  fibres  to  the  three  crests  of 
the  three  ampullae,  to  the  patch  on  the  utricle,  and  to  the 
patch  on  the  saccule.  In  each  crest  and  each  patch  the 
epithelium  is  thickened  and  modified,  and  although  the  crests 
are  slightly  different  in  structure  from  the  patches,  the  general 
features  are  the  same  in  all.  Whereas  over  the  rest  of  the 
inside  of  the  membranous  labyrinth  the  epithelium  consists 
(Fig.  1 26,  A,  e)  of  a  single  layer  of  low,  rather  fiat  cells,  in  the 
crests  and  patches  the  cells  lie  several  deep,  and  are  of  a  pecul- 
iar form.  Like  the  cells  in  the  olfactory  epithelium,  they  are 
of  two  kinds.  Some  are  columnar  and  bear  each  a  stiff, 
hairlike  filament  projecting  into  the  cavity  of  the  labyrinth 
(Fig.  126,  c.c,  a.h).  These  filaments,  often  called  auditory 
hairs,  appear  at  first  sight  to  resemble  cilia,  but  they  are  stiff, 
and,  unlike  cilia,  have  no  active  movement  of  their  own. 
They  are  longer  and  more  conspicuous  in  the  crests  of  the 
ampulla;  than  in  the  patches  of  the  utricle  and  saccule. 
The  other  cells  of  the  epithelium  of  the  crests  and  patches 
are  long  and  slender  bodies  with  ?.  bulging  nucleus  and  no 
hair,  and  are  probably  only  supporting  in  function  (sp.  c). 
The  fibres  of  the  auditory  nerve  may  be  traced  through  the 
connective  tissue  wall  of  the  crest  or.  patch  into  the  epi- 
thelium, where  they  break  up  into  delicate  filaments,  which 
appear  to  end,  not  in  the  cells,  but  among  them  (n,  a,  b). 

It  is  very  clear  that  movements  in  the  endolymph  may 
set  in  motion  these  hairs,  very  much  as  waves  of  the  wind 
set  in  motion  stalks  of  standing  grain,  and  that  the  move- 
ments of  the  hairs,  by  help  of  the  cells  to  which  the  hairs 
belong,  may  excite  the  delicate  nervous  filaments  and  so  set 
up  disturbances  or  impulses  which  pass  along  the  auditory 
nerve  to  the  brain.  It  is  probable,  as  we  shall  learn  more 
fully  later,  that  the  utricle,  saccule,  and  canals  are  not  con- 


ELEMENTARY    PHYSTOLOHY 


Fig.  126.  —  Diagrams  to  show  the  Structure  of  the  Crista  Acustica. 

A,  Longitudinal  section  of  ampulla,  the  crest  being  cut  crosswise. 

c,  one  end  of  the  ampulla  opening  into  the  semicircular  canal;  u,  the  other  end 
opening  into  the  utricle;  e,  ordinary  epithelium  lining  the  greater  part  of  the  ampulla; 
cr,  the  crest  with  a.e,  auditory  epithelium;  a./i,  auditory  hairs;  c.t,  connective 
tissue  support  to  the  auditory  epithelium;  n,  fibres  of  the  auditory  nerve  passing  into 
the  auditory  epithelium;  i,  epithelium  intermediate  between  the  auditory  epithelium 
and  the  ordinary  epithelium  of  the  rest  of  the  ampulla. 

B,  Diagram  to  illustrate  the  character  of  the  cells  of  the  auditory  epithelium  and 
the  relation  of  the  auditory  hairs  to  the  cells.  I,  the  auditory  epithelium;  II,  the 
connective  tissue  on  which  it  rests;  c.c,  cylindrical  cells  bearing  auditory  hair,  a.h; 
sp.c,  supporting  cells,  not  bearing  hairs. 

it,  a  fibre  of  the  auditory  nerve  passing  through  II  and  dividing  into  fine  branch- 
ing filaments  at  b. 


ix  THE   MEMBRANOUS   COCHLEA  399 

cerned  specifically  with  the  function  of  hearing,  but  have 
other  totally  different  functions. 

In  the  utricle  and  saccule,  where,  as  has  been  said,  the 
hairs  are  not  so  conspicuous,  a  mass  of  small  calcareous 
particles,  called  otoliths,  imbedded  in  a  soft  substance,  lies 
in  contact  with  the  tips  of  the  hairs.  In  some  of  the  lower 
animals  these  minute  particles  are  replaced  by  one  large 
stone. 

(ii)  The  Membranous  Cochlea.  —  An  important  part  of 
the  membranous  labyrinth  remains  to  be  described,  and 
that  is  the  cochlea,  which,  as  we  shall  see,  is  the  specifically 
auditory  part  of  the  ear. 

Connected  with  the  saccule  by  a  narrow  canal  is  an  exten- 
sion of  the  original  membranous  sac,  in  the  form  of  a  long 
tube,  closed  at  the  end  (Fig.  125,  Cocli).  This  cochlear 
tube,  like  the  parts  of  the  sac  already  described,  is  lined 
with  epithelium,  contains  endolymph,  and  is  lodged  in  a 
bony  cavity  filled  with  perilymph.  So  far  it  resembles  the 
rest  of  the  labyrinth,  but  in  many  other  respects  it  is  very 
different. 

In  the  first  place,  in  the  semicircular  canals  the  mem- 
branous walls  follow,  in  general,  the  contour  of  the  bony 
walls,  so  that  in  a  section  the  membranous  canal  presents  a 
flattened  circular  contour  lying  in  the  larger  circular  contour 
of  the  bony  canal.  But  in  the  cochlea,  on  the  contrary,  the 
contour  of  the  cochlear  tube  is,  along  its  whole  length,  to- 
tally different  from  that  of  the  containing  cavity  ;  for,  in 
transverse  section,  the  contour  of  the  containing  cavity  is 
almost  circular,  a  bony  ledge,  the  spiral  lamina,  projecting 
from  the  bony  wall  upon  one  side  for  a  certain  distance  into 
the  cavity  (Fig.  128,  /.s)  ;  but  the  section  of  the  cochlear 
tube  itself  is  nearly  triangular  (C.C).  The  cochlear  tube 
in  fact  is,  in  shape,  what  is  often  called  triangular  (as  when 


4oo  ELEMENTARY   PHYSIOLOGY  less. 

we  speak  of  a  triangular  file),  but  should  be  called  trihedral: 
that  is  to  say,  it  has  three  sides  or  faces  (and  three  edges). 

In  the  second  place,  in  the  utricle  and  saccule,  the  sac  is 
for  the  most  part  free  from  the  bony  walls,  being  attached 
only  at  the  places  where  the  nerve  fibres  pass  into  it,  and, 
more  loosely,  at  some  few  other  points  ;  but  in  the  cochlea, 
on  the  contrary,  the  cochlear  tube  closely  adheres  to  the 
bony  wall,  along  the  whole  length  of  the  tube,  in  two  regions, 
namely,  over  one  face  and  at  the  edge  opposite.  The  one 
face  is  attached  firmly  to  one  side  of  the  bony  wall,  and  the 
opposite  edge  adheres  to  the  projecting  edge  of  the  spiral 
lamina.  Thus  the  cochlear  tube,  containing  endolymph, 
together  with  the  spiral  lamina,  divides  the  cavity  contain- 
ing perilymph,  in  which  it  lies,  into  two  passages,  called 
scalse,  which  are  seen  in  section  (Fig.  127)  to  be  placed 
one  above  and  the  other  below  the  triangular  cavity  of  the 
cochlear  tube  itself.  The  membranous  tube  is  a  trifle  shorter 
than  the  bony  one,  hence  the  two  scalse  communicate  with 
each  other  at  the  far  end  of  the  tube,  but  not  elsewhere. 

In  the  third  place,  the  cochlear  tube  is  not  straight  or 
even  simply  curved,  but  is  twisted  upon  itself,  into  a  spiral 
of  two  and  a  half  turns.  In  these  twists  it  is  accompanied 
by  the  scalae  and  also  by  the  spiral  lamina,  whence  the  name 
of  the  latter  (Figs.  127,  L.S,  128,  l.s).  The  whole  arrange- 
ment somewhat  resembles  the  shell  of  a  snail ;  hence  the 
name  cochlea.  All  along  the  spiral  the  edge  of  the  cochlear 
tube  attached  to  the  lamina  spiralis  is  directed  inwards  and 
the  attached  face  outwards ;  so  that  when  a  section  is  made 
through  the  axis  of  the  spiral  a  succession  of  rounded  spaces 
is  cut  through,  each  space  exhibiting,  above  and  below,  the 
somewhat  half-moon-shaped  section  of  a  scala,  the  two  scalse 
being  separated,  on  the  outer  side,  by  the  cochlear  tube,  and, 
on  the  inner,  by  the  spiral  lamina  (Fig.  127). 


IX 


THE   MEMBRANOUS   COCHLEA 


401 


The  triangular  membranous  tube  which,  as  we  have  seen, 
contains  endolytnph  and  is  continuous  with  the  saccule,  is 
called  the  canal  of  the  cochlea,  or  scala  media  (because  it 
lies  between  the  two  other  scala;).  The  upper  of  the  two 
cavities  containing  perilymph,  when  traced  down  to  the 
bottom  of  the  spiral,  is  found  to  be  continuous  with  the 
cavity  containing  perilymph  which  surrounds  the  utricle 
and  saccule  and  is  called  the  vestibule;  hence  the  upper 
scala  is  called  the  scala  vestibuli.     The  lower  cavity,  when 


So-4t  j'0.p- 


Fig.  127.  —  A  Section  through  the  Axis  of  the  Cochlea,  magnified  Three 
Diameters. 

Sc.M,  scala  media;  Sc.V,  scala  vestibuli;  Sc.T,  scala  tympani;  L.S,  lamina 
spiralis;  Md,  bony  axis,  or  modiolus,  round  which  the  scalae  are  wound;  C.N, 
cochlear  nerve. 

similarly  traced  to  the  bottom  of  the  spiral,  ends  against  the 
inner  wall  of  the  middle  ear  or  tympanum  by  an  opening, 
called  the  fenestra  rotunda,  which  is  closed  by  a  membrane. 
Hence  this  lower  cavity  is  called  the  scala  tympani.  Thus, 
the  scala  vestibuli  and  scala  tympani  begin  at  different 
points,  and  are  separated  along  their  whole  course  by  the 
cochlear  tube  and  the  spiral  lamina,  except  at  the  very  tip 
of  the  spiral,  where  these  latter  end  ;  here  the  two  scalae 
are^  prolonged  beyond  the  cochlear  tube  and  join  together, 
forming  a  common  space,  as  seen  at  the  top  of  Fig.  127. 

The  vibrations  of  sound  are  brought,  as  we  shall  see,  to 
the  perilymph  chamber  of  the  vestibule,  whence  they  spread 


}D2  ELEMENTARY   PHYSIOLOGY  less. 

/nto  the  scala  vestibuli.  Passing  upwards  in  the  spiral  along 
the  scala  vestibuli,  they  enter  at  the  summit  the  scala  tym- 
pani,  along  which  they  descend,  and  are  eventually  lost  at 
the  fenestra  rotunda  in  which  that  scala  ends. 

(iii)  The  Organ  of  Corti.  —  But  besides  this  peculiar  ar- 
rangement of  the  chambers,  there  are  other  and  still  more 
important  differences  between  the  cochlea  and  the  rest  of 
the  labyrinth. 

The  auditory  nerve  is,  as  we  have  seen,  distributed  to 
certain  parts  only  of  the  rest  of  the  membranous  labyrinth, 
namely,  to  the  crests  of  the  ampulla?  and  to  the  patches  on 
the  utricle  and  the  saccule ;  but,  in  the  case  of  the  cochlea, 
fibres,  running  in  canals  excavated  in  the  bony  core  of  the 
spiral,  and  in  the  spiral  lamina  (Fig.  128,  AN),  run  to  and 
end  in  the  canal  of  the  cochlea  along  its  whole  length,  from 
the  bottom  to  the  top  of  the  spiral  (Fig.  125,  Coch).  And 
the  mode  of  ending  of  these  nerves  is  very  peculiar. 

If  we  examine  a  section  of  one  of  the  spirals  of  the  cochlea 
(Fig.  128),  we  see  that  the  upper  side  of  the  cochlear  tube 
(that  which  separates  it  from  the  scala  vestibuli)  is  formed 
by  a  thin  membrane  (called  the  membrane  of  Reissner,  Fig. 
128,  mR),  lined  internally  by  simple  epithelium.  The  outer 
convex  side  of  the  cochlear  tube,  that  side  by  which  it  is 
firmly  attached  to  the  bony  wall,  is  also  lined  internally  by 
simple  epithelium.  Neither  here  nor  in  the  membrane  of 
Reissner  do  any  fibres  of  the  auditory  nerve  end.  But  the 
remaining  side  of  the  tube,  that  which  looks  towards  the  scala 
tympani,  possesses  on  its  inner  face,  along  the  whole  length 
of  the  tube,  from  the  bottom  to  the  top  of  the  spiral,  a  very 
remarkable  and  strangely  modified  epithelium  ;  and,  along 
the  whole  length  of  the  tube,  fibres  of  the  auditory  nerve 
pass  to  and  end  among  the  cells  of  this  epithelium,  which  is 
spoken  of  as  the  organ  of  Corti  (Fig.  128,  O.C). 


THE    MEMBRANOUS    COCHLEA 


403 


The  membrane  which  separates  the  cavity  of  the  cochlear 
tube  from  the  scala  tympani,  and  on  which  the  organ  of 


Fig.  128.  —  Section  of  Coil  of  Cochlea. 

Sc.V,  scala  vestibuli;  Sc.T,  scala  tympani:  C.C,  canalis  cochlearis,  or  scala 
media;  O.C,  organ  of  Corti;  >«R,  membrane  of  Reissner:  »it,  membrana  tectoria 
(a  gelatinous  membrane  overlying  the  organ  of  Corti,  and  supposed  to  act  as  a 
damper).  AN,  fibres  of  the  auditory  nerve  running  in  l.s,  the  lamina  spiralis,  and 
ending  in  the  organ  of  Corti:  a,  connective  tissue  cushion  to  which  the  basilar  mem- 
brane is  attached  on  the  outside:  b,  bony  walls. 

The  figure  has,  for  simplicity's  sake,  been  made  somewhat  diagrammatic.  The 
spiral  lamina  has  been  drawn  too  short;  the  proportions  of  the  spiral  lamina  and  the 
seals  are  more  exactly  rendered  in  Fig.  127. 


404 


ELEMENTARY   PHYSIOLOGY 


Corti  is  placed,  is  of  a  peculiar  character,  consisting  of  thou- 
sands of  delicate  fibres  placed  side  by  side  and  extending 
across  the  canal ;  it  is  called  the  basilar  membrane.  The 
organ  of  Corti  itself  consists  of,  in  the  first  place,  the  so- 
called  rods  of  Corti,  peculiarly  shaped  long  bodies,  which 
are  seen  in  section  leaning,  as  it  were,  against  each  other. 
There  is  an  inner  row  of  these  and  an  outer  row  all  along 
the  spiral,  each  row  consisting  of  several  (four  to  six)  thou- 
sands of  rods.    At  the  inner  side  and  at  the  outer  side  of  the 


Fig.  129. — Transverse  Section  through  the  Side  Walls  of  the  Skull  to 
show  the  Parts  of  the  Ear;  the  Left  Ear  seen  from  in  front.  (After 
Arnold.)     (From  Quain's  Anatomy.) 

1,  Pinna;  2  to  2',  external  auditory  meatus;  2',  tympanic  membrane;  3,  cavity  of 
the  middle  ear;  above  3  the  chain  of  small  bones;  4,  Eustachian  tube;  5,  internal 
auditory  meatus,  containing  the  auditory  (lower)  and  facial  nerves  coming  from  the 
brain;  6,  bony  labyrinth  of  interna]  ear;  a,  petrous  part  of  temporal  bone;  c,  e,f, 
other  parts  of  temporal  bone;  b,  internal  carotid  artery;   d,  facial  nerve. 


rods  are  very  peculiar  epithelial  cells,  also  arranged  in  rows, 
each  row  consisting  of  several  thousand  cells.  Each  of  these 
cells  bears  short  hairs  on  its  free  surface,  hence  they  are  called 
hair-cells,  inner  and  outer.     The  fibres  of  the  auditory  nerves 


THE   BONY    LABYRINTH 


405 


passing  through  the  spiral  lamina  reach  the  cochlear  tube 
along  the  whole  length  of  the  spiral,  and  branch  into  fila- 
ments which  go  to  the  organ  of  Corti  and  terminate  among, 
but  probably  not  in,  the  hair-cells. 

11.  The  Bony  Labyrinth.  —  It  will  be  remembered 
that  the  membranous  labyrinth,  filled  with  endolymph,  lies 
in  an  intricate  cavity  with  bony  walls  called  the  osseous 
labyrinth  (Fig.  129,  6),  which  corresponds  to  the  former 
largely  but  not  wholly  in  form.  The  bony  vestibule  contains 
the  membranous  saccule  and  utricle ;  the  bony  semicircular 


Fig.  130.  —  The  Membrane  of  the  Drum  of  the  Right  Ear,  with  the  Small 
Bones  of  the  Ear  seen   from   the   Inner  Side;  and  the   Walls  of  the 
Tympanum,  with  the  Air-cells  in  the   Mastoid  Part  of  the  Temporal 
Bone. 
The  petrous  part  of  the  temporal  bone  containing  the  labyrinth  is  supposed  to  be 

removed,  the  foot-plate  of  the  stapes  having  been  detached  from  the  fenestra  ovalis. 
M.C,   mastoid  cells;  Mall,  malleus;   Inc,  incus;    St,  stapes;  a  b,   lines  drawn 

through  the  horizontal  axis  on  which  the  malleus  and  incus  turn. 


canals  contain  the  membranous  semicircular  canals  ;  the 
bony  cochlea,  with  its  scala  vestibuli  and  scala  tympani,  con- 
tains the  membranous  canal  of  the  cochlea,  or  scala  media. 
Between  the  membranous  walls  and  the  bony  walls  is  a  space 
filled  with  perilymph.  The  cavities  of  the  osseous  labyrinth 
are  chambers  in  the  petrous  part  of  the  temporal  bone. 
,In  the  living  body,  this  collection   of  chambers   in   the 


406  ELEMENTARY   PHYSIOLOGY  less. 

petrous  bone  is  perfectly  closed  ;  but,  in  the  dry  skull,  there 
are  two  wide  openings,  termed  fenestrae,  or  windows,  in  its 
outer  wall ;  i.e.  on  the  side  nearest  the  outside  of  the  skull 
and  between  the  internal  and  middle  ears.  Of  these  fenes- 
trae, one,  termed  ovalis  (the  oval  window)  (Fig.  131,  F.o.), 
is  situated  in  the  wall  of  the  vestibular  cavity ;  the  other, 
rotunda  (the  round  window)  F.r.,  behind  and  below  this, 
is,  as  we  have  seen,  the  open  end  of  the  scala  tympani  at  the 
base  of  the  spiral  of  the  cochlea.  In  the  living  body,  each 
of  these  windows  or  fenestrae  is  closed  by  a  fibrous  mem- 
brane, continuous  with  the  periosteum  of  the  bone. 

The  fenestra  rotunda  is  closed  by  membrane  only ;  but 
fastened  to  the  centre  of  the  membrane  of  the  fenestra  ovalis, 
so  as  to  leave  only  a  narrow  margin,  is  an  oval  plate  of  bone, 
part  of  one  of  the  little  bones  to  be  described  shortly. 

12.  The  Middle  Ear.  —  The  outer  wall  of  the  internal 
ear  is  still  far  away  from  the  exterior  of  the  skull.  Between 
it  and  the  visible  opening  of  the  ear,  in  fact,  are  placed  in 
a  straight  line,  first,  the  drum  of  the  ear  or  tympanum ; 
secondly,  the  long  external  passage,  or  meatus  (Fig.  129). 

The  drum  of  the  ear,  which  constitutes  the  middle  ear, 
and  the  external  meatus  would  form  one  cavity,  were  it 
not  that  a  delicate  membrane,  the  tympanic  membrane 
(Fig.  129,  2'),  is  tightly  stretched  in  an  oblique  direction 
across  the  passage,  so  as  to  divide  the  comparatively  small 
cavity  of  the  drum  from  the  meatus. 

The  membrane  of  the  tympanum  thus  prevents  any  com- 
munication, by  means  of  the  meatus,  between  the  drum  and 
the  external  air,  but  such  a  communication  is  provided, 
though  in  a  roundabout  way,  by  the  Eustachian  tube  (Fig. 
129,  4),  which  leads  directly  from  the  fore  part  of  the  drain 
inwards  to  the  roof  of  the  pharynx,  where  it  opens.  (See 
also  Fig.  76,  #) 


THE   AUDITORY    OSSICLES 


4C7 


(i)  The  Auditory  Ossicles.  —  Three  small  bones,  the  au- 
ditory ossicles,  lie  in  the  cavity  of  the  tympanum.  One 
of  these  is  the  stapes,  a  small  bone  shaped  like  a  stirrup. 
It  is  the  foot-plate  of  this  bone  which,  as  already  mentioned, 
is  firmly  fastened  to  the  membrane  of  the  fenestra  ovalis, 
while  its  hoop  projects  outwards  into  the  tympanic  cavity 
(Fig.  130,  St.,  and  Fig.  131,  Stp.). 


Fig.  131. — A  Diagram  illustrative  of  the  Relative  Positions  of  the  Vari- 
ous Parts  of  the  Ear. 

EM,  external  auditory  meatus;  Ty.M,  tympanic  membrane;  Ty,  tympanum; 
Mall,  malleus;  Inc,  incus;  Stp,  stapes;  F.o,  fenestra  ovalis;  F.r,  fenestra  rotunda; 
Eu,  Eustachian  tube;  M.L,  membranous  labyrinth,  only  one  semicircular  canal  with 
its  ampulla  being  represented;  Sca.V,  Sca.T,  Sea. ill,  the  scalse  of  the  cochlea, 
which  is  supposed  to  be  unrolled. 


Another  of  these  bones  is  the  malleus  {Mall.,  Figs.  130, 
131),  or  hammer- bone,  a  long  process,  the  so-called  handle 
of  which  is  fastened  to  the  inner  side  of  the  tympanic  mem- 
brane ;  while  a  very  much  smaller  process,  the  s  fender  process, 
is  fastened,  as  is  also  the  body  of  the  malleus,  to  the  bony 
wall  of  the  tympanum  by  ligaments.     The  rounded  surface 


4o8  ELEMENTARY   PHYSIOLOGY  les^ 

of  the  head  of  the  malleus  fits  into  a  corresponding  hollowed 
surface  in  the  end  of  a  third  bone,  the  incus,  or  anvil-bone, 
thus  forming  a  joint  of  a  somewhat  peculiar  character.  The 
incus  has  two  processes  ;  of  these  one.  the  shorter,  is  hori- 
zontal, and  rests  upon  a  support  afforded  to  it  by  the  walls 
of  the  tympanum  ;  while  the  other,  the  longer,  is  vertical, 
descends  almost  parallel  with  the  long  process  of  the  malleus, 
and  articulates1  with  the  stapes  {Inc.,  Figs.  130  and  131). 

The  three  bones  thus  form  a  movable  chain  between  the 
fenestra  ovalis  and  the  tympanic  membrane.  The  malleus 
and  incus  are,  by  the  peculiar  joint  spoken  of  above,  articu- 
lated together  in  such  a  manner  that  they  may  practically 
be  considered  as  forming  one  bone  which  turns  upon  a  hori- 
zontal axis.  This  axis  passes  through  the  horizontal  process 
of  the  incus  and  the  slender  process  of  the  malleus,  and  its 
ends  rest  in  the  walls  of  the  tympanum.  Its  general  direc- 
tion is  represented  by  the  line  ab  in  Fig.  130,  or  by  a  line 
perpendicular  to  the  plane  of  the  paper,  passing  through  the 
head  of  the  malleus,  in  Fig.  131. 

The  two  bones  may  be  roughly  compared  to  two  spokes 
of  a  wheel,  of  which  the  axle  is  represented  by  the  axis  just 
described  ;  it  should  be  added,  however,  that  one  spoke,  the 
incus,  is  shorter  than  the  other,  and  that  the  movement  of 
the  two  spokes  is  limited  to  a  very  small  arc  of  a  circle. 

When  the  membrane  of  the  drum,  thrown  into  vibration 
by  some  sound,  moves  inwards  and  outwards  in  its  vibrations, 
it  necessarily  carries  with  it,  in  each  inward  and  outward 
movement,  the  handle  of  the  malleus  which  is  attached  to  it. 
But  with  each  inward  and  outward  movement  of  the  handle 

1  A  minute  hone,  the  os  orbiculare,  intervenes  between  the  end  of  the  pro- 
cess of  the  incus  and  the  stapes,  so  that  the  stapes  is  in  reality  articulated 
with  the  os  orbiculare,  which  in  turn  is  fastened  to  the  process  of  the  incus. 
For  simplicity's  sake,  mention  of  this  is  omitted  above. 


ix  THE   MUSCLES   OF  THE  TYMPANUM  409 

of  the  malleus,  the  long  process  of  the  incus  also  moves  in- 
wards and  outwards,  carrying  with  it  the  stapes  which  is 
attached  to  its  end.  Hence  each  vibration,  each  inward 
thrust,  and  each  outward  or  backward  return  of  the  mem- 
brane of  the  drum,  produces  by  means  of  the  chain  of 
ossicles  a  corresponding  vibration  of  the  membrane  of  the 
fenestra  ovalis  to  which  the  stapes  is  attached ; 1  but  the 
vibrations  of  this  membrane  are  in  turn  communicated  to 
the  perilymph  of  the  labyrinth  and  cochlea.  Thus,  by  means 
of  the  chain  of  ossicles  and  the  membranes  to  which  these  are 
attached  at  each  end,  the  aerial  vibrations  passing  down  the 
meatus  are  transformed  into  corresponding  vibrations  of  the 
fluids  of  the  inner  ear.  The  vibrations  of  the  perilymph 
passing  up  the  scala  vestibuli,  and  down  the  scala  tympani, 
reach  at  last  the  membrane  covering  the  fenestra  rotunda 
and  throw  this  into  vibration  ;  and  as  a  matter  of  fact  it  has 
been  observed  that  when  the  membrane  of  the  fenestra 
ovalis  moves  inwards,  that  of  the  fenestra  rotunda  moves  out- 
wards, and  vice  versa. 

The  vibrations  of  the  perilymph  thus  produced  will  affect 
the  endolymph,  and  thus  the  hairs,  and  so  the  auditory  epi- 
thelium of  the  labyrinth  ;  by  which,  finally,  the  auditory 
nerves  will  be  excited. 

(ii)  The  Muscles  of  the  Tympanum. — The  characters  of 
the  vibration  of  a  membrane,  and  the  readiness  with  which 
it  takes  up  or  responds  to  aerial  vibrations  reaching  it,  are 
largely  modified  by  its  degree  of  tension  ;  the  membrane 
acts   differently  when  it  is  tightly  stretched  from  what  it 

1  Owing  to  certain  characters  in  the  attachment  of  the  stapes  to  the  mem- 
brane of  the  fenestra  ovalis  on  the  one  hand,  and  to  the  os  orbiculare  on 
the  other,  the  movements  of  the  foot  of  the  stapes  in  the  fenestra  ovalis  are 
somewhat  peculiar;  but  the  details  of  these  as  well  as  the  functions  of  the 
peculiar  articulation  of  the  incus  with  the  malleus  have,  for  simplicity's  sake, 
been  omitted. 


4io  ELEMENTARY   PHYSIOLOGY  less. 

does  when  it  is  loose.  Now,  within  the  cavity  of  the 
tympanum  are  two  small,  but  relatively  strong  muscles. 
One,  called  the  stapedius,  passes  from  the  floor  of  the 
tympanum  to  the  foot  of  the  stapes  and  the  orbicular  bone, 
the  other,  the  tensor  tympani,  from  the  front  wall  of  the 
drum  to  the  malleus.  Each  of  the  muscles  when  it  con- 
tracts tightens  the  membrane  to  which  it  is  thus  indirectly 
attached,  the  tensor  tympani,  the  membrane  of  the  drum, 
and  the  stapedius,  the  membrane  of  the  fenestra  ovalis. 
The  effect  of  thus  tightening  the  membrane  is  probably  to 
restrict  the  vibrations  of  the  membrane,  at  least  as  far  as 
concerns  grave,  or  low-pitched  sounds ;  but  the  complete 
action  of  these  muscles  is  too  intricate  to  be  dwelt  on  here. 

13.  The  External  Ear. — The  outer  extremity  of  thi 
external  meatus  is  surrounded  by  the  pinna,  the  two  together 
constituting  the  external  ear  (Fig.  129,  1).  The  pinna  is  a 
broad,  peculiarly  shaped,  and  for  the  most  part  cartilagi- 
nous plate,  the  general  plane  of  which  is  at  right  angles  with 
that  of  the  axis  of  the  auditory  opening.  The  pinna  can  be 
moved,  by  most  animals  and  by  some  human  beings,  in 
various  directions  by  means  of  muscles,  which  pass  to  it 
from  the  side  of  the  head. 

14.  The  Transmission  of  Sound  Waves  to  the  Inner 
Ear.  — The  manner  in  which  the  complex  apparatus  now 
described  intermediates  between  the  physical  agent,  which 
is  the  primary  cause  of  the  sensation  of  sound,  and  the 
nervous  expansion,  the  affection  of  which  alone  can  excite 
that  sensation,  must  next  be  considered. 

All  bodies  which  produce  sound  are  in  a  state  of  vibration, 
and  they  communicate  the  vibrations  of  their  own  substance 
to  the  air  with  which  they  are  in  contact,  and  thus  throw 
that  air  into  waves,  just  as  a  stick  waved  backwards  and 
forwards  in  water  throws  the  water  into  waves. 


ix  TRANSMISSION  OF   SOUND   WAVES  411 

The  aerial  waves,  produced  by  the  vibrations  of  sonorous 
bodies,  in  part  enter  the  external  auditory  passage,  and  in 
part  strike  upon  the  pinna  of  the  external  ear  and  the  outer 
surface  of  the  head.  It  may  be  that  some  of  the  latter 
impulses  are  transmitted  through  the  solid  structure  of  the 
skull  to  the  organ  of  hearing ;  but  before  they  reach  it  they 
must,  under  ordinary  circumstances,  have  become  so  scanty 
and  weak,  that  they  may  be  left  out  of  consideration. 

The  aerial  waves  which  enter  the  meatus  all  impinge  upon 
the  membrane  of  the  drum  and  set  it  vibrating,  stretched 
membranes,  especially  such  as  have  the  form  and  characters 
of  the  tympanic  membrane,  taking  up  vibrations  from  the 
air  with  great  readiness. 

The  vibrations  thus  set  up  in  the  membrane  of  the 
tympanum  are  communicated,  in  part,  to  the  air  contained 
in  the  drum  of  the  ear,  and,  in  part,  to  the  malleus,  and 
thence  to  the  other  auditory  ossicles. 

The  vibrations  communicated  to  the  air  of  the  drum 
impinge  upon  the  inner  wall  of  the  tympanum,  on  the 
greater  part  of  which,  from  its  density,  they  can  produce 
very  little  effect.  Where  this  wall  is  formed  by  the  mem- 
brane of  the  fenestra  rotunda  the  communication  of  motion 
must  necessarily  be  greater.  All  these  vibrations,  however, 
may  probably  be  neglected. 

The  vibrations  which  are  communicated  to  the  malleus 
and  the  chain  of  ossicles  may  be  of  two  kinds  :  vibrations 
of  the  particles  of  the  bones,  and  vibrations  of  the  bones  as 
a  whole.  If  a  beam  of  wood,  freely  suspended,  be  very 
gently  scratched  with  a  pin,  its  particles  will  be  thrown  into 
a  state  of  vibration,  as  will  be  evidenced  by  the  sound  given 
out,  but  the  beam  itself  will  not  be  visibly  moved.  Again, 
if  a  strong  wind  blow  against  the  beam,  it  will  swing  bodilv, 
without   any  vibrations   of  its  particles   among  themselves. 


412  ELEMENTARY   PHYSIOLOGY  less. 

On  the  other  hand,  if  the  beam  be  sharply  struck  with  a 
hammer,  it  will  not  only  give  out  a  sound,  showing  that  its 
particles  are  vibrating,  but  it  will  also  swing,  from  the 
impulse  given  to  its  whole  mass. 

Under  the  last-mentioned  circumstances,  a  blind  man 
standing  near  the  beam  would  be  conscious  of  nothing  but 
the  sound,  the  product  of  molecular  vibration,  or  invisible 
oscillation  of  the  particles  of  the  beam  ;  while  a  deaf  man  in 
the  same  position  would  be  aware  of  nothing  but  the  visible 
oscillation  of  the  beam  as  a  whole. 

Thus,  to  return  to  the  chain  of  auditory  ossicles,  while  it 
may  be  supposed  that,  when  the  membrane  of  the  drum 
vibrates,  these  may  be  set  vibrating  both  as  a  whole  and  in 
their  particles,  the  question  arises  whether  it  is  the  large 
vibrations,  or  the  minute  ones,  which  make  themselves  obvi- 
ous to  the  auditory  nerve,  which  is  in  the  position  of  our 
deaf,  or  blind,  man. 

The  evidence  is  distinctly  in  favour  of  the  conclusion, 
that  it  is  the  vibrations  of  the  bones,  as  a  whole,  which  are 
the  chief  agents  in  transmitting  the  impulses  of  the  aerial 
waves. 

For,  in  the  first  place,  the  disposition  of  the  bones  and 
the  mode  of  their  articulation  are  very  much  against  the 
transmission  of  molecular  vibrations  through  their  substance, 
but,  on  the  other  hand,  are  extremely  favourable  to  their 
vibration  en  masse.  The  long  processes  of  the  malleus  and 
incus  swing,  like  a  pendulum,  upon  the  axis  furnished  by  the 
short  processes  of  these  bones ;  while  the  mode  of  connec- 
tion of  the  incus  with  the  stapes,  and  of  the  latter  with  the 
membrane  of  the  fenestra  ovalis,  allows  the  foot-plate  of  that 
bone  free  play,  inwards  and  outwards.  In  the  second  place, 
the  total  length  of  the  chain  of  ossicles  is  very  small  com- 
pared with  the  length  of  the  waves  of  audible  sounds,  and 


ix  TRANSMISSION   OF   SOUND   WAVES  413 

physical  considerations  teach  us  that  in  a  like  thin  rod, 
similarly  capable  of  swinging  en  masse,  the  minute  molecular 
vibrations  would  be  inappreciable.  Thirdly,  direct  experi- 
ments, such  as  attaching  to  the  stapes  of  a  dissected  ear  a 
light  style,  the  movements  of  which  are  recorded  on  a 
travelling  smoked  glass  plate  or  in  some  other  way,  show 
that  the  chain  of  ossicles  does  actually  vibrate  as  a  whole, 
and  at  the  same  rate  as  the  membrane  of  the  drum,  when 
aerial  vibrations  strike  upon  the  latter. 

Thus,  there  is  reason  to  believe  that  when  the  tympanic 
membrane  is  set  vibrating,  it  causes  the  process  of  the 
malleus,  which  is  fixed  to  it,  to  swing  at  the  same  rate ;  the 
head  of  the  malleus  consequently  turns  through  a  small  arc 
on  its  pivot,  the  slender  process.  But,  as  stated  on  p.  408, 
the  turning  of  the  head  of  the  malleus  involves  the  simultane- 
ous turning  of  the  head  of  the  incus  upon  its  pivot,  the  short 
process.  In  consequence  the  long  process  of  the  incus  also 
swings  at  the  same  rate.  The  length  of  the  long  process  of 
the  incus,  measured  from  the  axis,  on  which  the  two  bones 
turn,  is  less  than  that  of  the  handle  of  the  malleus ;  hence 
the  end  of  it  moves  through  a  smaller  space.  The  arc 
through  which  it  moves  has  been  estimated  as  being  equal 
to  about  two-thirds  of  that  described  by  the  handle  of  the 
malleus.  The  extent  of  the  push  is  thereby  somewhat 
diminished,  but  the  force  of  the  push  is  proportionately 
increased  ;  in  so  confined  a  space  this  change  is  advantage- 
ous. The  long  process  of  the  incus,  however,  is  so  fixed  to 
the  stapes,  and  the  stapes  so  attached  to  the  membrane  of 
the  fenestra  ovalis,  that  the  incus  cannot  vibrate  without 
throwing  into  vibrations,  to  a  corresponding  extent  and  at 
the  same  rate,  the  membrane  of  the  fenestra  ovalis.1  But 
every  vibration,  every  pull  and  push,  imparts  a  correspond- 

1  See  foot-note,  p.  408. 


414  ELEMENTARY   PHYSIOLOGY  less. 

ing  set  of  shakes  to  the  perilymph,  which  fills  the  bony 
labyrinth  external  to  the  membranous  labyrinth.  These 
shakes  are  communicated  to  the  endolymph  in  the  latter 
chamber,  and,  by  the  help  of  the  modified  auditory  epithe- 
lium described  above,  stimulate  the  delicate  endings  of  at 
least  the  cochlear  division  of  the  auditory  nerve. 

15.  The  Conversion  of  Sonorous  Vibrations  into  Sensa- 
tions of  Sound.  —  We  do  not  at  present  know  what  kind  of 
changes  the  vibrations  of  the  endolymph  give  rise  to  in  the 
epithelial  cells  of  the  organ  of  Corti ;  nor  do  we  at  present 
know  the  exact  way  in  which  the  changes  thus  set  up  in 
these  epithelial  cells  are  able  to  excite  the  terminal  filaments 
of  the  auditory  nerve.  But  there  can  be  no  doubt  of  the 
fact  that  the  elaborate  apparatus  of  the  cochlea  is  able  to 
translate,  so  to  speak,  the  sonorous  vibrations  which  reach 
them  into  stimulations  of  nerve-fibres,  the  molecular  changes 
of  which  are  transmitted  along  the  auditory  nerve  as  audi- 
tory nervous  impulses.  Passing  along  the  auditory  nerve, 
these  molecular  changes,  these  nervous  impulses,  reach  cer- 
tain parts  of  the  brain  situated  in  the  cortex  of  the  temporo- 
sphenoidal  lobe,  below  the  fissure  of  Sylvius  (see  p.  550), 
and  there  in  turn  set  up  those  molecular  disturbances  of 
nervous  matter  which  form  the  immediate  cause  of  the  states 
of  feeling  called  "sounds."  Thus,  the  auditory  nerve  may 
be  said,  and  a  similar  statement  may  be  made  in  the  case 
of  the  other  nerves  of  special  sensations,  to  be  provided 
with  two  "end-organs."  There  is  the  peripheral  end-organ 
(the  apparatus  of  the  cochlea)  by  which  the  physical  agent 
is  enabled  to  excite  the  sensory  nerve-fibres ;  and  there  is 
the  central  end-organ,  in  the  brain,  in  which  the  nervous 
impulses  of  the  sensory  nerve  excite  the  special  state  of  feel- 
ing which  we  call  the  special  sensation.  The  central  end- 
organ  of  hearing  is  often  spoken  of  as  the  auditory  sensorium. 


ix  ACTION   OF  THE  AUDITORY  END-ORGANS  415 

Between  the  sounding  body  and  the  actual  hearing  of  a 
sound,  there  is  a  chain  of  events  of  different  kinds.  There 
are  the  vibrations  started  by  the  sounding  body,  and  pass- 
ing through  the  air,  the  tympanum,  the  perilymph,  and  the 
endolymph ;  these  are  all  of  one  order.  Then  there  are 
the  changes  in  the  peripheral  end-organ,  in  the  apparatus  of 
the  cochlea  ;  these  are  of  another  order.  Then  follow  the 
molecular  disturbances  travelling  along  the  auditory  nerve ; 
these  are  of  still  another  order.  Lastly,  there  are  the 
changes  in  the  central  end-organ,  in  the  brain ;  these,  though 
resembling  the  preceding  in  so  far  as  they  are  changes  of 
nervous  matter,  are  yet  of  still  another  order,  and  probably 
comprise  in  themselves  a  whole  series  of  events,  the  conse- 
quence of  the  last  of  which  is  the  sensation  of  sound. 

16.  The  Mode  of  Action  of  the  Auditory  End-organs. — 
Every  sound  consists,  as  we  have  seen,  of  vibrations.  Some- 
times the  vibrations  are  repeated  with  great  regularity ;  and 
sounds,  in  which  the  regular  recurrence  of  the  same  vibra- 
tions is  conspicuous,  are  called  "musical  sounds."  Some- 
times no  regular  repetition  of  vibrations  can  be  recognised  ; 
the  sound  consists  of  vibrations,  few  of  which  are  like  each 
other,  and  which  fall  irregularly  on  the  ear ;  such  sounds  are 
called  "  noises." 

When  we  listen  to  musical  sounds,  each  set  of  regularly 
repeated  vibrations  generates  in  the  central  end-organ  a 
particular  kind  of  sensation  which  we  call  a  tone;  and  the 
simultaneous  or  successive  production  of  different  tone- 
sensations  gives  rise  in  us  to  the  feelings  which  we  speak  of 
as  those  of  harmony  or  melody. 

When  we  listen  to  a  noise  the  vibrations  generate  sensa- 
tions which  are  of  a  certain  intensity,  according  to  which 
we  call  the  noise  slight  or  great,  low  or  loud,  and  which  also 
have  certain  characters  by  which  we  recognise  the  kind  of 


4i6  ELEMENTARY   PHYSIOLOGY  less. 

noise  ;  but  the  sensations  have  not  the  qualities  of  tone-sen 
sations,  and  do  not  give  rise  to  feelings  of  melody  or  har- 
mony. 

A  pure  musical  sound  consists  of  a  series  of  vibrations 
repeated  with  exact  regularity,  the  number  of  vibrations 
occurring  in  a  given  time,  e.g.  in  a  second,  determining  what 
is  called  the  pitch  of  the  "  note."  But  ordinary  musical 
sounds  are,  for  the  most  part,  not  simple,  consisting  of  one 
set  of  vibrations,  but  compound,  consisting  of  several  sets  of 
vibrations  occurring  together;  in  these  musicians  distinguish 
one  set,  called  the  fundamental  tone,  and  other  sets,  vary- 
ing in  intensity  or  loudness,  called  overtones. 

A  tuning-fork,  when  set  vibrating,  vibrates  with  a  given 
rapidity  ;  and  the  note  given  out  is'determined  by  the  rapid- 
ity of  the  vibration,  by  the  number  of  vibrations  repeated, 
for  instance,  in  a  second  ;  hence  every  tuning-fork  has  its 
own  proper  note.  Now,  a  tuning-fork  will  be  set  vibrating 
if  its  own  particular  note  be  sounded  in  its  neighbourhood, 
but  not  if  other  notes  be  sounded.  Hence,  when  a  pure 
musical  note  is  sounded  close  to  a  number  of  tuning-forks 
of  different  pitch,  only  that  tuning-fork  the  pitch  of  which  is 
the  same  as  that  of  the  note  sounded  is  set  vibrating; 
the  others  remain  motionless.  When  an  ordinary  musical 
sound,  such  as  a  note  sung  by  the  human  voice,  is  produced 
among  such  a  group  of  tuning-forks,  several  are  set  vibrat- 
ing ;  one  of  these  corresponds  to  the  fundamental  tone,  and 
the  others  to  the  various  overtones  of  the  sound.  Similarly, 
if  the  top  of  a  piano  be  lifted  up  or  removed,  and  any  one 
sings  into  the  wires  with  sufficient  loudness  a  note,  such  as 
the  tenor  c,  a  number  of  the  wires  will  be  set  vibrating,  one 
corresponding  to  the  fundamental  tone,  and  the  others  to 
the  overtones. 

If  we  were  to  imagine  an  immense  number  of  tuning-forks, 


ix  ACTION  OF  THE  AUDITORY  END-ORGANS  417 

each  vibrating  at  different  periods,  so  arranged  that  each 
fork,  when  vibrating,  in  some  way  or  other  stimulated  or 
excited  a  minute  delicate  nerve  filament  attached  to  it,  it  is 
obvious  that  a  musical  sound  uttered  near  these  tuning- 
forks  wouhd  set  a  certain  number  of  them  into  vibration, 
some  more  forcibly  than  others,  and  that  in  consequence  a 
certain  number,  and  a  certain  number  only,  of  the  delicate 
nerve  filaments  would  be  excited,  and  that  to  various  de- 
grees ;  and  thus  a  particular  series  of  nervous  impulses,  the 
counterpart  as  it  were  of  the  musical  sound  with  its  funda- 
mental tone  and  overtones,  would  be  transmitted  along  the 
nerve  filaments  to  the  brain. 

It  is  suggested  that  the  basilar  membrane  of  the  cochlea, 
consisting  as  it  does  of  thousands  of  fibres  stretching  across 
from  the  inside  to  the  outside  (from  left  to  right  in  Fig. 
128),  with  its  thousands  of  epithelial  cells  and  rods  of  Corti 
lying  upon  it,  represents,  as  it  were,  an  assemblage  of  thou- 
sands of  tuning-forks,  of  various  rates  of  vibration,  with  a 
separate  nerve  filament  adapted  to  each.  So  that,  when  a 
number  of  vibrations  of  different  periods,  such  as  consti- 
tutes an  ordinary  musical  sound,  are  transmitted  by  the 
tympanum  to  the  cochlea,  these,  as  they  sweep  along  the 
canal  of  the  cochlea,  throw  into  sympathetic  movement 
those  parts,  and  those  parts  only,  of  the  basilar  membrane 
with  their  overlying  epithelium  and  rods  of  Corti  whose 
periods  of  vibration  correspond  to  the  incoming  vibrations, 
and  thus  excite  certain  nerve  filaments,  and  these  only.  It 
is  this  excitement  of  a  group  of  nerve  filaments,  some  ex- 
cited more  intensely  than  others,  which,  reaching  the  brain, 
gives  rise  to  the  sensation  which  we  associate  with  a  particu- 
lar musical  sound. 

We  know  something  in  general  about  the  position  in  the 
brain  of  the  auditory  sensorium  or  central  end-organ  of  the 


418  ELEMENTARY   PHYSIOLOGY  less. 

auditory  nerve  ;  but  we  know  very  little  about  the  nature  of 
this  sensorium.  It  may  be  conceived,  however,  that  each 
filament  of  the  cochlear  nerve  is  connected  with  a  particu- 
lar portion  of  the  nervous  matter  of  the  central  end-organ, 
in  such  a  way  that  the  molecular  movements  of  one  of  these 
particular  portions  of  nervous  matter,  brought  about  by  a 
molecular  disturbance  reaching  it  through  its  appropriate 
filament,  produces  a  psychical  effect  of  one  kind  only,  more 
or  less  intense  it  may  be,  but  still  always  of  one  kind.  If 
this  be  so,  each  cochlear  fibre  or  filament  may  be  considered 
as  being  provided  with  two  end-organs  :  one,  peripheral,  in 
the  organ  of  Corti,  capable  of  being  set  in  motion  by  vibra- 
tions of  one  quality  only ;  the  other,  central,  in  the  brain, 
capable  of  producing  a  psychical  effect  of  one  quality  only. 
It  does  not  follow,  however,  that  we  are  distinctly  and  sepa- 
rately conscious  of  the  nervous  disturbance  in  each  central 
end-organ,  it  does  not  follow  that  we  have  as  many  distinct 
and  separate  kinds  of  conscious  sensation  as  there  are  periph- 
eral and  central  end-organs,  though  how  many  such  dis- 
tinct kinds  of  sensation  we  may  have  we  do  not  know.  Just 
as  the  peripheral  mechanism  sifts  out  the  several  vibrations 
of  which  a  musical  sound  is  composed,  and  transmits  them 
separately,  so,  by  a  reverse  operation,  the  central  mechanism 
probably  pieces  together  the  nervous  disturbances  of  a  num- 
ber of  central  end-organs,  and  thus  produces  a  sensation 
whose  characters  are  determined  by  a  combination  of  the 
nervous  disturbances  taking  place  in  each  end-organ. 

Some  such  a  view  is  indeed  exceedingly  probable  ;  but  it 
must  be  remembered  that  we  do  not  at  present  at  all  under- 
stand the  exact  mechanism  by  which  each  particular  vibra- 
tion excites  its  corresponding  nerve  filament.  The  nerve 
filaments  appear  to  end  among  the  epithelial  cells  bearing 
short  hairs,  which  lie  on  each  side  of  the  rods  of  Corti ;  and 


ix  LOCALISATION   OF   SOUND  419 

we  may,  therefore,  conclude  that  these  "  hair-cells "  have 
some  share  in  producing  the  effect  and  constitute  the  essen- 
tial part  of  the  organ  of  hearing.  But  the  whole  matter  is 
at  present  very  obscure  ;  the  functions  of  the  rods  of  Corti 
are  particularly  difficult  to  understand  ;  for  these  do  not 
seem  in  any  way  connected  with  the  nerve  filaments,  and 
their  movements  can  only  affect  the  latter  by  influencing  in 
some  way  the  hair-cells. 

The  fibres  of  the  cochlear  nerve,  or  their  endings  in  the 
brain  itself,  may  be  excited  by  internal  causes,  such  as  the 
varying  pressure  of  the  blood  and  the  like  :  and  in  some 
persons  such  internal  influences  do  give  rise  to  sensations  of 
sounds  and  even  to  veritable  musical  spectra,  sometimes  of 
a  very  intense  character.  But,  for  the  appreciation  of  music 
produced  external  to  us,  we  depend  upon  the  organ  of  Corti 
being  in  some  way  or  other  affected  by  the  vibrations  of  the 
fluids  in  the  cochlea. 

It  has  been  suggested  that  the  utricle,  saccule,  and  semi- 
circular canals  enable  us  to  appreciate  noises  \  but  such  a 
view  presents  great  difficulties.  Between  noises  and  musi- 
cal sounds  no  hard  and  fast  line  can,  in  fact,  be  drawn.  It 
seems  probable  that  the  cochlea  deals  with  both  kinds  of 
sonorous  vibrations. 

17.  Localisation  of  Sound. — The  apparatus  of  the  ear 
which  we  have  described,  provides  us  simply  with  auditory 
sensations ;  enables  us  to  appreciate  high  notes  and  low 
notes,  to  discriminate  between  musical  sounds  and  noises. 
Experience  then  enables  us  to  base  upon  these  sensations 
certain  conclusions  as  to  the  nature  of  the  source  which  is 
giving  rise  to  each  sound.  But  sounds  may  be  coming  to 
us  in  different  directions  and  from  different  distances,  and 
when  we  endeavour  to  form  some  estimate  of  either  the  one 
or  the  other  of  these  possible  differences,  we  find  that  our  means 


420  ELEMENTARY   PHYSIOLOGY  less. 

of  doing  so  are  very  imperfect.  As  to  our  estimate  of  the 
distance  from  which  a  sound  is  coming,  we  are  guided 
chiefly  by  its  intensity  coupled  with  previous  experience. 
For  the  discrimination  of  the  direction  from  which  a  sound 
is  coming,  we  have  to  rely  almost  entirely  on  the  different 
effect  the  sound  produces  on  each  of  our  two  ears,  accord- 
ing as  it  falls  more  directly  into  one  of  them  than  into  the 
other.  Thus  when  we  are  endeavouring  to  localise  a  source 
of  sound,  we  usually  turn  the  head  into  various  positions, 
until  we  find  one  position  in  which  the  sound  is  loudest  as 
it  falls  into  one  ear,  and  then  we  assume  that  the  sound  is 
coming  along  a  line  directed  straight  into  that  ear.  In  ani- 
mals with  large  and  movable  external  ears,  the  movement  of 
the  ear  to  a  great  extent  takes  the  place  of  the  movement 
of  the  head ;  this  may  be  readily  observed  in  an  animal 
such  as  the  horse. 

Anything  which  interferes  with  the  ordinary  laws  of 
transference  of  sound  causes  us  to  form  a  wrong  judgment 
as  to  the  distance  of  the  source,  as  in  the  case  of  listening 
to  speech  through  a  telephone  or  in  a  phonograph. 
Similarly,  it  is  difficult  to  estimate  the  distance  of  the 
source  of  a  sound  heard  through  a  snow  storm.  Again, 
in  ventriloquism  our  judgment  is  upset,  not  only  as 
regards  the  nature  of  the  source  of  sound,  but  also  of  its 
distance  and  direction,  by  carefully  planned  simulation  and 
suggestion. 

18.  The  Functions  of  the  Tympanic  Muscles  and 
E:\stacbian  Tube.  —  It  has  already  been  explained  that 
the  stapedius  and  tensor  tympani  muscles  are  competent 
to  tighten  the  membrane  of  the  fenestra  ovalis  and  that  of 
the  tympanum  respectively,  and  it  is  probable  that  they 
come  into  action  when  the  sonorous  impulses  are  too  vio- 
lent, and  would  produce  too  extensive  vibrations  of  the«;e 


IX         FUNCTIONS   OF  THE   SEMICIRCULAR   CANALS       421 

membranes.  They  may  therefore  be  of  use  in  moderating 
the  effect  of  intense  sound,  in  much  the  same  way  that,  as 
we  shall  find,  the  contraction  of  the  circular  fibres  of  the  iris 
tends  to  moderate  the  effect  of  intense  light  in  the  eye ; 
they  may,  however,  have  other  purposes. 

The  function  of  the  Eustachian  tube  is,  probably,  to 
keep  the  air  in  the  tympanum,  or  on  the  inner  side  of  the 
tympanic  membrane,  of  about  the  same  tension  as  that  on 
the  outer  side,  which  could  not  always  be  the  case  if  the 
tympanum  were  a  closed  cavity.  The  unpleasant  sensa- 
tion often  experienced,  as  of  a  "  tightness "  in  the  ear, 
when  diving  under  water,  is  due  to  the  compression  of 
the  air  in  the  tympanic  cavity  under  the  increased  external 
pressure.  It  may  be  largely  removed  by  merely  performing 
the  movements  of  swallowing.  By  these  movements  the 
end  of  the  Eustachian  tube  which  opens  into  the  pharynx 
is  opened  and  the  pressure  on  the  two  sides  of  the  tym- 
panum is  equalised. 

19.  The  Functions  of  the  Semicircular  Canals,  the 
Utricle,  and  the  Saccule.  —  It  is  probable  that  the  semi- 
circular canals,  the  utricle,  and  the  saccule  have  nothing  to 
do  with  hearing,  and  it  is  known  that  they  have  other  very 
definite  functions,  namely,  that  of  enabling  the  body  to 
maintain  its  equilibrium. 

We  have  seen  that  the  semicircular  canals  lie  in  three 
planes  at  right  angles  to  one  another  (p.  395).  When  any 
one  of  the  canals  is  experimentally  injured,  the  animal  in 
many  cases  executes  a  series  of  oscillatory  movements  of 
the  head,  which  are,  broadly  speaking,  in  the  plane  of  the 
canal.  When  all  three  canals  are  injured,  the  animal  is 
thrown  into  continuous  movements  of  the  most  varied  and 
often  extraordinary  kind,  and  has  lost  all  power  of  balancing 
itself  in  a   normal    way.     Not    unfrequently  in   man   these 


422  ELEMENTARY   PHYSIOLOGY  less,  ix 

canals  undergo  injury  as  the  result  of  disease,  and  in  this 
case  the  feelings  experienced  by  the  patient  are  those  of 
extreme  giddiness,  and  an  inability  to  balance  the  body, 
while  the  symptoms  exhibited  to  an  onlooker  are  those  of  a 
want  of  co-ordination  in  the  execution  of  movements.  Thus, 
there  is  no  doubt  that  the  canals  enable  us  to  appreciate 
the  movements  of  the  head  in  all  planes  in  space,  and  thus 
act  as  sense-organs  for  the  guidance  of  our  bodily  move- 
ments. A  movement  of  the  head  causes  a  change  of 
pressure  in  the  endolymph,  and  thus  the  hair-cells  of  the 
crista;  are  stimulated.  It  is  a  suggestive  fact  that  the 
canals  are  relatively  largest  in  animals,  such  as  birds  and 
fishes,  that  live  in  a  fluid  medium  rather  than  upon  the 
ground,  and  whose  locomotor  movements  are  often  sudden 
and  delicate. 

Some  movements  of  the  body  also  are  apparently  appre- 
ciated by  means  of  the  utricle  and  saccule,  but  these  parts 
of  the  labyrinth  seem,  in  addition,  to  give  us  notions  of  the 
position  of  the  resting  body  in  space.  Probably  the  con- 
stant pressure  of  the  otoliths  on  the  hair-cells  of  the  macula; 
acts  as  a  constant  stimulus,  the  pressure  being  varied  accord- 
ing to  the  position  in  which  the  body  rests,  whether  upright, 
lying  down,  etc. 

These  various  organs  doubtless  act  together  and  enable 
us  to  control  all  our  bodily  movements  very  perfectly  and 
thus  to  maintain  our  equilibrium  under  all  circumstances. 
The  vestibular  branch  of  the  auditory  nerve,  which  supplies 
these  organs,  is  distinct  from  the  cochlear  branch,  and, 
instead  of  ending  with  the  latter  in  that  part  of  the  brain 
that  has  to  do  with  hearing,  goes  to  the  cerebellum,  which, 
as  we  shall  see,  has  as  its  function  the  co-ordination  of  bodily 
movements. 


LESSON   X 

THE    ORGAN    OF   SIGHT 

1.  The  General  Structure  of  the  Eye.  —  In  studying 
the  organ  of  the  sense  of  sight,  the  eye,  we  may,  perhaps 
with  advantage,  consider  the  accessory  parts  first,  and  then 
pass  on  to  the  essential  structures. 

The  accessory  organs,  by  means  of  which  the  physical 
agent  of  vision,  light,  is  enabled  to  act  upon  the  expansion  of 
the  optic  nerve,  comprise  three  kinds  of  apparatus  :  (a)  a 
"water  camera,"  the  eyeball;  (b)  muscles  for  moving  the 
eyeball ;  (r)  organs  for  protecting  the  eyeball,  viz.  the  eye- 
lids, with  their  lashes,  glands,  and  muscles  ;  the  conjunctiva ; 
and  the  lachrymal  gland  and  its  ducts. 

The  ball,  or  globe,  of  the  eye  is  a  globular  body,  moving 
freely  in  a  chamber,  the  orbit,  which  is  furnished  to  it  by 
the  skull.  The  optic  nerve,  the  root  of  which  is  in  the  brain, 
leaves  the  skull  by  a  hole  at  the  back  of  the  orbit,  and  enters 
the  back  of  the  globe  of  the  eye,  not  in  the  middle,  but  on 
the  inner,  or  nasal,  side  of  the  centre.  Having  pierced  the 
wall  of  the  globe,  it  spreads  out  into  a  very  delicate  mem- 
brane, varying  in  thickness  from  ^  of  an  inch  to  less  than 
half  that  amount,  which  lines  the  hinder  two-thirds  of  the 
globe,  and  is  termed  the  retina.  This  retina  is  the  only  or- 
gan connected  with  sensory  nervous  fibres  which  can  be 
affected,  by  any  agent,  in  such  a  manner  as  to  give  rise  to 
the  sensation  of  light.     It  contains  the  essential  part  of  the 

423 


424  ELEMENTARY   PHYSIOLOGY  less 

organ  of  vision,  the  rods  and  cones,  and  the  one  pre-eminent 
function  of  the  accessory  structures  is  to  bring  the  rays  of 
light  entering  the  eye  from  external  objects  to  a  focus  upon 
the  rods  and  cones. 

The  eyeball  is  composed,  in  the  first  place,  of  a  tough, 
firm,  spheroidal  case  consisting  of  fibrous  tissue,  the  greater 
part  of  which  is  white  and  opaque,  and  is  called  the  sclerotic 
(Fig.  132,  2).  In  front,  however,  this  fibrous  capsule  of  the 
eye,  though  it  does  not  change  its  essential  character,  be- 
comes transparent,  and  receives  the  name  of  the  cornea 
(Fig.  132,  1).  The  front  surface  of  the  cornea  is  covered 
by  an  epithelium,  in  which  the  cells  are  very  similar  and 
similarly  arranged  to  those  in  the  epidermis  of  the  skin.  The 
corneal  portion  of  the  case  of  the  eyeball  is  more  convex  than 
the  sclerotic  portion,  so  that  the  whole  form  of  the  ball  is 
such  as  would  be  produced  by  cutting  off  a  segment  from 
the  front  of  a  spheroid  of  the  diameter  of  the  sclerotic,  and 
replacing  this  by  a  segment  cut  from  a  smaller,  and  conse- 
quently more  convex,  spheroid. 

The  corneo-sclerotic  case  of  the  eye  is  kept  in  shape  by 
what  are  termed  the  humours  —  watery  or  semi-fluid  sub- 
stances, one  of  which,  the  aqueous  humour  (Fig.  132,  7'), 
which  is  hardly  more  than  water  holding  a  few  organic  and 
saline  substances  in  solution,  distends  the  corneal  chamber 
of  the  eye,  while  the  other,  the  vitreous  humour  (Fig.  132, 
13),  which  is  rather  a  delicate  jelly  than  a  regular  fluid, 
keeps  the  sclerotic  chamber  full. 

The  two  humours  are  separated  by  the  very  beautiful, 
transparent,  doubly  convex  crystalline  lens  (Fig.  132,  12), 
denser,  and  capable  of  refracting  light  more  strongly  than 
either  of  the  humours.  The  crystalline  lens  is  composed  of 
fibres  having  a  somewhat  complex  arrangement,  and  is  highly 
elastic.     It  is  more  convex  behind  than  in  front,  and  it  is 


GENERAL   STRUCTURE   OF  THE  EYE 


425 


kept  in  place  by  a  delicate,  but  at  the  same  time  strong 
membranous  frame  or  suspensory  ligament,  which  extends 
from  the  edges  of  the  lens  to  what  are  termed  the  ciliary- 
processes  of  the  choroid  coat  (Figs.  132,  5,  and  134,  c). 
In  the  ordinary  condition  of  the  eye  this  ligament  is  kept 

—  C^ULiM  JuUM 


r..l  bruua- 


dudkflrldu 


Fig.  132. 


-  Horizontal  Section  of  the  Eyeball. 


1,  cornea;  i',  conjunctiva;  2,  sclerotic;  2',  sheath  of  optic  nerve;  3,  choroid; 
3",  rods  and  cones  of  the  retina;  4,  ciliary  muscle;  4',  circular  portion  of  ciliary 
muscle;  5,  ciliary  process;  6,  posterior  chamber  between  7,  the  iris,  and  10,  the  sus- 
pensory ligament:  7',  anterior  chamber;  8,  artery  of  retina  in  the  centre  of  the  optic 
nerve;  8',  centre  of  blind  spot;  8",  macula  lutea;  9,  ora  serrata  (this  is  of  course  not 
seen  in  a  section  such  as  this,  but  is  introduced  to  show  its  position) ;  10,  the  sus- 
pensory ligament;  12,  crystalline  lens;  13,  vitreous  humour;  14,  space  in  tissue  called 
the  canal  of  Schlemm;  a  a,  optic  axis;  b  b,  line  of  equator  of  the  eyeball. 

tense,  i.e.  is  stretched  pretty  tight,  and  the  front  part  of  the 
lens  is  consequently  flattened. 

The  choroid  coat  (Fig.  132,  3)  is  highly  vascular  and 
consists  of  blood-vessels  arranged  in  a  very  complex  way, 


426 


ELEMENTARY    PHYSIOLOGY 


bound  together  with  a  little  connective  tissue  among  which, 
towards  its  outer  side,  are  a  number  of  branched  connective- 
tissue  corpuscles  whose  cell-substance  is  loaded  with  gran- 
ules of  black  pigment  (Fig.  133). 

The  choroid  is  in  close  contact  with  the  sclerotic  exter- 
nally, and  internally  is  in  contact  with  a  layer  of  very 
peculiar  cells,  also  full  of  pigment  (Fig.  145)  belonging  to 
the  retina.  The  choroid  lines  every  part  of  the  sclerotic, 
except  just  where  the  optic  nerve  enters  it  at  a  point  below, 
and  to  the  inner  side  of  the  centre  of  the  back  of  the  eye ; 


Fig.  133.  —  Pigment  Cells  from  the  Choroid  Coat. 


but,  when  it  reaches  the  front  part  of  the  sclerotic,  its  inner 
surface  becomes  raised  up  into  a  number  of  longitudinal 
ridges,  with  intervening  depressions,  like  a  fluted  ruffle,  ter- 
minating within  and  in  front  with  rounded  ends,  but  passing, 
externally,  into  the  iris.  These  ridges,  which  when  viewed 
from  behind  seem  to  radiate  on  all  sides  from  the  lens 
(Figs.  134,  c,  and  132,  5),  are  the  above-mentioned  ciliary 
processes. 

The  iris  itself  (Figs.  132,  7,  and  134,  a,  &)  is,  as  has 
been  already  said  (p.  324),  a  curtain  with  a  round  hole  in 
the  middle,  the  piipil,  provided  with  circular  and  radiating 


X        GENERAL  STRUCTURE  OF  THE  EYE       427 

unstriped  muscular  fibres,  and  capable  of  having  its  central 
aperture  diminished  or  enlarged  by  the  action  of  these 
■fibres,  the  contraction  of  which,  unlike  that  of  other 
unstriped  muscular  fibres,  is  extremely  rapid.  The  hinder 
surface  of  the  iris  is  covered  with  cells  containing  a  black 
pigment,  similar  to  that  of  the  choroid  coat,  and  the  differ- 
ent colours  of  eyes  depend  partly  on  the  varying  amount  and 
distribution  of  pigment  in  these  cells,  and  partly  on  pigment 
cells  imbedded  in  and  scattered  throughout  the  substance 
of  the  iris.  The  outer  edges  of  the  iris  are  continuous  with 
the  choroid.  Unstriped  muscular  fibres,  originating  in  the 
sclerotic  at  its  junction  with  the  cornea,  spread  backwards 
on  to  the  outer  surface  of  the  choroid  and  constitute  the 
ciliary  muscle  (Fig.  132,  4).  If  these  fibres  contract,  it  is 
obvious  that  they  will  pull  the  choroid  forwards ;  and  as  the 
frame  or  suspensory  ligament  of  the  lens  is  connected  with 
the  ciliary  processes  (which  simply  form  the  anterior  ter- 
mination of  the  choroid),  this  pulling  forward  of  the  choroid 
comes  to  the  same  thing  as  a  relaxation  of  the  tension  of 
that  suspensory  ligament,  which,  as  we  have  just  said,  is  in 
an  ordinary  condition  stretched  somewhat  tight,  keeping 
the  front  of  the  lens  flattened. 

The  iris  does  not  hang  down  perpendicularly  into  the 
space  between  the  front  face  of  the  crystalline  lens  and 
the  posterior  surface  of  the  cornea,  which  is  filled  by 
the  aqueous  humour,  but  applies  itself  very  closely  to  the 
anterior  face  of  the  lens,  so  that  hardly  any  interval  is  left 
between  the  two  (Figs.  132  and  137). 

The  retina,  the  structure  of  which  will  be  considered  later. 
lines  the  interior  of  the  eye,  being  placed  between  the  choroid 
and  vitreous  humour,  its  rods  and  cones  being  imbedded  in 
the  pigment  epithelium  lying  just  within  the  former,  and  its 
inner  limiting  membrane  touching  the  latter  (Fig.  132,  3"). 


428  ELEMENTARY  PHYSIOLOGY  less. 

About  a  third  of  the  distance  back  from  the  front  of  the 
eye  the  retina  seems  to  end  in  a  wavy  border  called  the 
ora  serrata  (Fig.  132,  9),  and  in  reality  the  nervous  ele- 
ments of  the  retina  do  end  here,  having  become  consider- 
ably reduced  before  this  line  is  reached.  Some  of  the 
connective-tissue  elements,  however,  pass  on  as  a  delicate 
kind  of  membrane  at  the  back  of  the  ciliary  processes 
towards  the  crystalline  lens. 


Fig.  134. — View  of  Front  HalF'Of  the  Eyeball  seen  from  Behind. 

a,  circular  fibres;  b,   radiating  fibres  of  the  iris;   c,  ciliary  processes;  d,  choroid. 

The  crystalline  lens  has  been  removed. 

2.  The  Eye  as  a  Water  Camera. — The  impact  of  the 
vibrations  of  the  ether  upon  the  sensory  expansion,  or  essen- 
tial part  of  the  visual  apparatus,  alone  is  sufficient  to  give 
rise  to  all  those  feelings  which  we  term  sensations  of  light  and 
of  colour,  and,  further,  to  that  feeling  of  outness  which 
accompanies  all  visual  sensation.  But,  if  the  retina  had 
a  simple  transparent  covering,  the  vibrations  radiating  from 
any  number  of  distinct  luminous  points  in  the  external 
world  would  affect  all  parts  of  it  equally,  and  therefore  the 
feeling  aroused  would  be  that  of  a  generally  diffused  lumi- 
nosity. There  would  be  no  separate  feeling  of  light  for 
each  separate  radiating  point,  and  hence  no  correspondence 


x  THE  EYE  AS  A  WATER  CAMERA  429 

between  the  visual  sensations  and  the  radiating  points  which 
aroused  them. 

It  is  obvious  that  in  order  to  produce  this  correspond- 
ence, or,  in  other  words,  to  have  distinct  vision,  the  essential 
condition  is,  that  distinct  luminous  points  in  the  external 
world  shall  be  represented  by  distinct  feelings  of  light. 
And  since,  in  order  to  produce  these  distinct  feelings,  vibra- 
tions must  fall  on  separate  parts  of  the  retina,  it  follows  that, 
for  the  production  of  distinct  vision,  some  apparatus  must  be 
interposed  between  the  retina  and  the  external  world,  by  the 
action  of  which  distinct  luminous  points  in  the  latter  shall  be 
represented  by  corresponding  points  of  light  on  the  retina. 

In  the  eye  of  man  and  of  the  higher  animals,  this  acces- 
sory apparatus  of  vision  is  represented  by  structures  which, 
taken  together,  act  as  a  biconvex  lens,  composed  of  sub- 
stances which  have  a  much  greater  refractive  power  than 
the  air  by  which  the  eye  is  surrounded ;  and  which  throw 
upon  the  retina  luminous  points,  which  correspond  in 
number  and  in  position,  relatively  to  one  another,  with 
those  luminous  points  in  the  external  world  from  which 
ethereal  vibrations  proceed  towards  the  eye.  The  luminous 
points  thus  thrown  upon  the  retina  form  a  picture  of  the 
external  world  —  a  picture  being  nothing  but  lights  and 
shadows,  or  colours,  arranged  in  such  a  way  as  to  correspond 
with  the  disposition  of  the  luminous  parts  of  the  object 
represented,  and  with  the  qualities  of  the  light  which  pro- 
ceeds from  them. 

That  a  biconvex  lens  is  competent  to  produce  a  picture 
of  the  external  world  on  a  properly  arranged  screen  is  a 
fact  of  which  every  one  can  assure  himself  by  simple 
experiments.  An  ordinary  magnifying  glass  is  a  trans- 
parent body  denser  than  the  air,  and  convex  on  both 
sides.     If  this  lens  be  held  at  a  certain  distance  from  a 


+30  ELEMENTARY   PHYSIOLOGY  less. 

screen  or  wall  in  a  dark  room  ami  a  lighted  candle  be 
placed  on  the  opposite  side  of  it,  it  will  be  easy  to  adjust 
the  distances  of  candle,  lens  and  wall  in  such  a  manner 
that  an  image  of  the  flame  of  the  candle,  upside  down,  shall 
be  thrown  upon  the  wall. 

The  spot  on  which  the  image  is  formed  is  called  a  focus. 
If  the  candle  be  now  brought  nearer  to  the  lens,  the  image 
on  the  wall  will  enlarge,  and  grow  blurred  and  dim,  but 
it  may  be  restored  to  brightness  and  definition  by  moving 
the  lens  further  from  the  wall.  But  if,  when  the  new 
adjustment  has  taken  place,  the  candle  be  moved  away 
from  the  lens,  the  image  will  again  become  confused,  and, 
to  restore  its  clearness,  the  lens  will  have  to  be  brought 
nearer  the  wall. 

Thus  a  convex  lens  forms  a  distinct  picture  of  luminous 
objects,  but  only  at  the  focus  on  the  side  of  the  lens 
opposite  to  the  object ;  and  that  focus  is  nearer  when  the 
object  is  distant,  and  further  off  when  it  is  near. 

Suppose,  however,  that,  leaving  the  candle  unmoved, 
a  lens  with  more  convex  surfaces  is  substituted  for  the 
first,  the  image  will  be  blurred,  and  the  lens  will  have  to 
be  moved  nearer  the  wall  to  give  it  definition.  If,  on 
the  other  hand,  a  lens  with  less  convex  surfaces  is  sub- 
stituted for  the  first,  it  must  be  moved  further  from  the  wall 
to  attain  the  same  end. 

In  other  words,  other  things  being  alike,  the  more  convex 
the  lens,  the  nearer  its  focus  ;  the  less  convex,  the  further 
off  its  focus. 

If  the  lens  were  made  of  some  extensible,  elastic  sub- 
stance, like  india-rubber,  pulling  it  at  the  circumference 
would  render  it  flatter,  and  thereby  lengthen  its  focus; 
while,  when  let  go  again,  it  would  become  more  convex, 
and  of  shorter  focus. 


X  THE    EYE    AS    A    WATER   CAMERA  431 

Any  material  more  refractive  than  the  medium  in  which 
it  is  placed,  if  it  have  a  convex  surface,  causes  the  rays  of 
light  which  pass  through  the  less  refractive  medium  to  that 
surface  to  converge  towards  a  focus.  If  a  watch-glass  be 
fitted  into  one  side  of  a  box,  and  the  box  be  then  filled 
with  water,  a  candle  may  be  placed  at  such  a  distance  out- 
side the  watch-glass  that  an  image  of  its  flame  shall  fall  on 
the  opposite  wall  of  the  box.  If,  under  these  circumstances, 
a  doubly  convex  lens  of  glass  were  introduced  into  the  water 
in  the  path  of  the  rays,  it  would  (though  less  powerfully  than 
if  it  were  in  air)  bring  the  rays  more  quickly  to  a  focus, 
because  glass  refracts  light  more  strongly  than  water  does. 

A  camera  obscura  is  a  box,  into  one  side  of  which  a  lens 
is  fitted,  so  as  to  be  able  to  slide  backwards  and  forwards, 
and  thus  throw  on  the  screen  at  the  back  of  the  box  dis- 
tinct images  of  bodies  at  various  distances.  Hence  the 
arrangement  just  described  might  be  termed  a  water 
camera. 

The  eyeball,  the  most  important  constituents  of  which 
have  now  been  described,  is,  in  principle,  a  camera  of  the 
kind  described  above  — a  water  camera.  That  is  to  say, 
the  sclerotic  answers  to  the  box,  the  cornea  to  the  watch- 
glass,  the  aqueous  and  vitreous  humours  to  the  water  filling 
the  box,  and  the  crystalline  to  the  glass  lens,  the  introduc- 
tion of  which  was  imagined.  The  back  of  the  box  corre- 
sponds with  the  retina. 

But,  further,  in  an  ordinary  camera  obscura  it  is  found 
desirable  to  have  what  is  termed  a  diaphragm  (that  is,  an 
opaque  plate  with  a  hole  in  its  centre)  in  the  path  of  the 
rays,  for  the  purpose  of  moderating  the  light  and  cutting  off 
the  marginal  rays,  which,  owing  to  certain  optical  properties 
of  spheroidal  surfaces,  give  rise  to  defects  in  the  image 
formed  at  the  focus. 


£J2  ELEMENTARY   PHYSIOLOGY  less, 

In  the  eye,  the  place  of  this  diaphragm  is  taken  by  the 
iris,  which  has  the  peculiar  advantage  of  being  self-regu- 
lating :  contracting  its  aperture  and  admitting  less  light 
when  the  illumination  is  strong  ;  but  dilating  its  aperture 
and  admitting  more  light  when  the  light  is  weak.  It  thus 
acts  like  the  various  "  stops  "  which  a  photographer  uses 
according  to  the  varying  light. 

These  changes  in  the  pupil  are  brought  about  by  the  con- 
tractions of  the  circular  and  radiating  muscle-fibres  of  the 
iris;  contraction  of  the  circular  or  sphincter  fibres  makes 
the  pupil  smaller  or  constricts  it,  contraction  of  the  radiat- 
ing fibres  makes  it  larger  or  dilates  it.  Further,  conversely, 
relaxation  of  the  circular  fibres  causes  or  helps  to  cause  dila- 
tion, and  relaxation  of  the  radiating  fibres  causes  or  helps 
to  cause  constriction.  Contraction  of  the  circular  fibres 
and  so  constriction  of  the  pupil  are  brought  about  by  means 
of  fibres  of  the  oculo-moior  nerve,  and  contraction  of  the 
radiating  fibres  and  so  active  dilation  are  brought  about  by 
means  of  fibres  of  tne  sympathetic  system. 

The  constriction  of  the  pupil  observed  when  light  falls 
upon  the  retina  is  a  reflex  action  in  which  the  optic  nerve 
provides  the  path  for  afferent  impulses  to  a  centre  in  the 
brain  lying  beneath  the  front  end  of  the  aqueduct  of  Sylvius 
(p.  523),  and  the  third  (oculo-motor)  cranial  nerve  (p.  536) 
provides  the  path  for  efferent  impulses  from  the  centre  to 
the  circular  fibres  of  the  iris.  The  dilation  of  the  pupil 
when  light  is  withdrawn  from  the  retina  is,  in  the  main  at 
least,  due  to  the  cessation  of  previously  acting  constrictor 
impulses. 

The  pupil  is  also  constricted  when  the  eye  is  accommo- 
dated for  near  objects,  and  during  deep  sleep  ;  and  it  is 
dilated  when  the  eye  is  accommodated  for  distant  objects. 

Rays  of  light  coming  from  an  object  and  passing  into  the 


TIIK    MECHANISM    OF   ACCOMMODATION 


43J 


eye  undergo  a  bending  or  refraction  (i)  as  they  enter  the 
eye,  at  the  surface  of  the  cornea,  (ii)  as  they  pass  through 
the  lens  ;  and  as  a  result  of  this  action  of  the  cornea  and 
lens  an  image  of  the  object  is  formed  on  the  retina  (Fig. 

135)- 

In  the  water  camera  the  image  brought  to  a  focus  on  the 

screen   at   the  back  is  inverted ;  the  image   of  a  tree,  for 

instance,  is  seen  with  the  roots  upwards  and  the  leaves  and 

branches  hanging  downwards.     The  right  of  the  image  also 

corresponds  with  the   left   of  the   object,  and  vice  versa. 

Exactly  the   same   thing  takes   place  in  the    eye  with  the 


Fig.  135. — The  Formation  of  an  Image  on  the  Retina. 


image  focussed  on  the  retina.  It  too  is  inverted.  This  fact 
often  gives  rise  to  the  question,  Why  then  do  we  see  objects 
in  the  external  world  in  an  erect  position  and  not  also 
inverted?     This  matter  is  discussed  in  Lesson  XI,  p.  466. 

3.  The  Mechanism  of  Accommodation.  —  In  the  water 
camera,  constructed  according  to  the  description  given  above, 
there  is  the  defect  that  no  provision  exists  for  adjusting  the 
focus  to  the  varying  distances  of  objects.  If  the  box  were 
so  made  that  its  back,  on  which  the  image  is  supposed  to  be 
thrown,  received  distinct  images  of  very  distant  objects,  all 
near  ones  would  be  indistinct.  And  if,  on  the  other  hand, 
it  were  fitted  to  receive  the  image  of  near  objects,  at  a  given 

Z  F 


434  ELEMENTARY   PHYSIOLOGY  less. 

distance,  those  of  either  still  nearer,  or  more  distant,  bodies 
would  be  blurred  and  indistinct.  In  the  ordinary  camera 
this  difficulty  is  overcome  by  sliding  the  lenses  in  and  out,  a 
process  which  is  not  compatible  with  the  construction  of  our 
water  camera.  But  there  is  clearly  one  way  among  many  in 
which  this  adjustment  might  be  effected  —  namely,  by 
changing  the  glass  lens  ;  putting  in  a  less  convex  one  when 
more  distant  objects  had  to  be  pictured,  and  a  more  convex 
one  when  the  images  of  nearer  objects  were  to  be  thrown 
upon  the  back  of  the  box. 

But  it  would  come  to  the  same  thing,  and  be  much  more 
convenient,  if,  without  changing  the  lens,  one  and  the  same 
lens  could  be  made  to  alter  its  convexity.  This  is  what 
actually  is  done  in  the  adjustment  of  the  eye  to  distances. 

The  simplest  way  of  experimenting  on  the  adjustment  ox 
accommodation  of  the  eye  is  to  stick  two  stout  needles  up- 
right into  a  straight  piece  of  wood,  not  exactly,  but  nearly 
in  a  line  parallel  with  the  edge  of  the  piece,  so  that,  on 
applying  the  eye  to  one  end  of  the  piece,  one  needle 
(a)  shall  be  seen  about  six  inches  off,  and  the  other  (b)  just 
on  one  side  of  it  at  twelve  inches  or  more  distance. 

If  the  observer  look  at  the  needle  b,  he  will  find  that  he  sees 
it  very  distinctly,  and  without  the  least  sense  of  effort ;  but 
the  image  of  a  is  blurred  and  more  or  less  double.  Now  let 
him  try  to  make  this  blurred  image  of  the  needle  a  distinct. 
He  will  find  he  can  do  so  readily  enough,  but  that  the  act 
is  accompanied  by  a  sense  of  effort  somewhere  in  the  eye. 
And  in  proportion  as  a  becomes  distinct,  b  will  become 
blurred.  Nor  will  any  effort  enable  him  to  see  a  and  b  dis- 
tinctly at  the  same  time. 

Multitudes  of  explanations  have  been  given  of  this  re- 
markable power  of  adjustment ;  but  the  true  solution  of  the 
problem  has  been  gained  by  the  accurate  determination  of 


X  THE   MECHANISM    OF    ACCOMMODATION  435 

the  nature  of  the  changes  in  the  eye  which  accompany  the 
act.  When  the  flame  of  a  taper  is  held  near,  and  a  little  on 
one  side  of,  a  person's  eye,  any  one  looking  into  the  eye 
from  a  proper  point  of  view  will  see  three  images  of  the 
flame,  two  upright  and  one  inverted  (Fig.  136,  A).  One  up- 
right bright  image  is  reflected  from  the  front  of  the  cornea, 
which  acts  as  a  convex  mirror.  The  second,  less  bright, 
proceeds  from  the  front  of  the  crystalline  lens,  which  has 
the  same  effect ;  while  the  inverted  image,  which  is  small 
and  indistinct,  proceeds  from  the  posterior  face  of  the  lens, 


Fig.  136. — Diagram  of  the  Images  of  a  Candle-flame  seen  by  Reflection 
from  the  Surface  07  the  Cornea  and  the  Two  Surfaces  of  the  Lens. 

A,  as  seen  when  the  eye  is  adjusted  for  a  distant  object:   B,  as  they  appear  when  the 
eye  is  fixed  on  a  near  object. 

which,  being  convex  backwards,  is,  of  course,  concave  for- 
wards, and  acts  as  a  concave  mirror. 

Suppose  the  eye  to  be  steadily  fixed  on  a  distant  object, 
and  then  adjusted  to  a  near  one  in  the  same  line  of  vision, 
the  position  of  the  eyeball  remaining  unchanged.  Then  the 
upright  image  reflected  from  the  surface  of  the  cornea,  and 
the  inverted  image  from  the  back  of  the  lens,  will  remain 
unchanged,  though   it   is   demonstrable  that  their   size   01 


436 


ELEMENTARY   PHYSIOLOGY 


LESS 


apparent  position  must  change  if  either  the  cornea,  or  the 
back  of  the  lens,  alters  either  its  form  or  its  position 
But  the  second  upright  image,  that  reflected  by  the  front 
face  of  the  lens,  does  change  both  its  size  and  its  position ; 
it  comes  forward  and  grows  smaller  (Fig.  136,  B),  proving 
that  the  front  face  of  the  lens  has  become  more  convex. 
The  change  of  form  of  the  lens  is,  in  fact,  that  represented 
in  Fig.  137. 

A  I  b 


Fig.  137.  —  The  Changes  in  the  Lens  in  Accommodation. 

A,  adjusted  for  distant;   B,  for  near  objects. 

c,  cornea;  con,  conjunctiva;  scl,  sclerotic;   ch,  choroid;   c.fi,  ciliary  process;  cm, 

ciliary  muscle;  s.l,  suspensory  ligament. 


For  purposes  of  accurate  experiment  it  is  better  to 
employ  the  images  cast  by  two  small  luminous  points  placed 
one  above  the  other.  In  this  case  three  pairs  of  images  are 
seen  by  reflection ;  and  it  is  easier  to  observe  that  the  two 
images  of  the  middle  pair  come  nearer  together  when  the 
eye  is  accommodated  for  a  near  object  than  it  is  to  observe 
the  slight  movement  and  diminution  in  size  of  the  single 
image  of  a  candle  flame. 

These  may  be  regarded  as  the  facts  of  adjustment  with 
which  all  explanations  of  that  process  must  accord.  The 
following  explanation,  which  was  proposed  by  Helmholtz,  is 
the  most  generally  accepted  one.    It  seems  probable  from  the 


x  USE  OF   SPECTACLES  437 

anatomical  relations  of  the  parts,  and  it  is  supported  by 
direct  experimental  evidence.  The  lens,  which  is  very 
elastic,  is  kept  habitually  in  a  state  of  compression  by  the 
pressure  exerted  on  it  by  its  suspensory  ligament,  and  con- 
sequently has  a  flatter  form  than  it  would  take  if  left  to  it- 
self. If  the  ciliary  muscle  contracts,  it  must,  as  has  been 
seen,  relax  that  ligament,  and  thereby  diminish  its  pressure 
upon  the  lens.  The  lens,  consequently,  will  become  more 
convex ;  it  will,  however,  since  it  is  highly  elastic,  return  to 
its  former  shape  when  the  ciliary  muscle  ceases  to  contract 
and  allows  the  choroid  to  return  to  its  ordinary  place. 

Hence,  probably,  the  sense  of  effort  we  feel  when  we 
adjust  for  near  distances  arises  from  the  contraction  of  the 
ciliary  muscle. 

4.  The  Limits  of  Accommodation.  Use  of  Spectacles. 
—  Accommodation  can  take  place  only  within  a  certain 
range  ;  this,  however,  admits  of  great  individual  variations. 

People  possessing  ordinary,  or,  as  it  is  called,  "normal" 
sight  can  adjust  their  eyes  so  as  to  see  distinctly  objects  as 
near  to  the  eye  as  five  or  six  inches ;  but  the  image  of  an 
object  brought  nearer  than  this  becomes  blurred  and  indis- 
tinct, because  the  "near  limit"  of  accommodation  is  then 
passed.  They  can  also  adjust  their  eyes  for  objects  at  a  very  • 
great  distance,  the  indistinctness  of  the  images  of  objects 
very  far  off  being  due,  not  to  want  of  proper  focussing,  but 
to  the  details  being  lost  through  the  minuteness  of  the 
image. 

Some  people,  however,  are  born  with,  or  at  least  come  to 
possess,  eyes  in  which  the  "  near  limit  "  of  accommodation  is 
much  closer.  Such  persons  can  see  distinctly  objects  as 
near  to  the  cornea  as  even  one  or  two  inches ;  but  they  can- 
not adjust  their  eyes  to  objects  at  any  great  distance  off. 
Thus,  many  of  these  "near-sighted"  people,  as  they  are 


438  ELEMENTARY   PHYSIOLOGY  less. 

called,  cannot  see  distinctly  the  features  of  a  person  only 
a  few  feet  off.  Though  their  ciliary  muscle  remains  quite 
relaxed  so  that  the  suspensory  ligament  keeps  the  lens  as 
fiat  as  possible,  the  arrangements  of  the  eye  are  such  that 
the  image  of  an  object  only  a  few  feet  off  is  brought  to  a 
focus  in  front  of  the  retina,  somewhere  in  the  vitreous 
humour.  By  wearing  concave  glasses  these  near-sighted 
people  are  able  to  bring  the  image  of  distant  objects  on  to 
the  retina  and  thus  to  see  them  distinctly. 

The  cause  of  near-sightedness  is  not  always  the  same,  but 
in  the  majority  of  cases  it  appears  to  be  due  to  the  bulb  of 
the  eye  being  unusually  long  from  back  to  front.  If,  in  the 
water  camera  described  above,  when  the  lens  and  object 
were  so  adjusted  that  the  image  of  the  object  was  distinctly 
focussed  on  the  screen,  the.  box  were  made  longer,  so  that 
the  screen  was  moved  backwards,  the  distinctness  of  the 
image  on  it  would  be  lost. 

Some  people  are  born  really  "  long-sighted,"  inasmuch  as 
they  can  see  distinctly  only  such  objects  as  are  quite  dis- 
tant ;  and,  indeed,  have  to  contract  their  ciliary  muscles, 
and  so  make  their  lens  more  convex  even  to  see  these. 
Near  objects  they  cannot  see  distinctly  at  all  unless  they  use 
convex  glasses.  In  such  persons  the  bulb  of  the  eye  is 
generally  too  short. 

A  kind  of  long-sightedness  also  comes  on  in  old  people ; 
but  this  is  different  from  the  above,  and  is  simply  due,  in 
the  majority  of  cases  at  all  events,  to  a  loss  of  power  of 
adjustment.  The  refractive  power  of  the  eye  remains  the 
same,  but  the  ciliary  muscle  fails  to  work;  and  hence 
adjustment  for  near  objects  becomes  impossible,  though 
distant  objects  are  seen  as  before.  For  near  objects  such 
persons  have  to  use  convex  glasses.  They  should  perhaps 
be  called  "  old-sighted  "  rather  than  "  long-sighted." 


TIIK    MUSCLES   OF    THE    EYEBALL 


439 


5.  The  Muscles  of  the  Eyeball.  —  The  muscles  which 
move  the  eyeball  are  altogether  six  in  number  —  four  straight 
muscles,  or  recti,  and  two  oblique  muscles,  the  obliqui  (Fig. 
138).  The  straight  muscles  are  attached  to  the  back  of  the 
bony  orbit,  round  the  edges  of  the  hole  through  which  the 
optic  nerve  passes,  and  run  straight  forward  to  their  inser- 
tions into  the  sclerotic  —  one,  the  superior  rectus,  in  the 
middle  line  above  ;  one,  the  inferior,  opposite  it  below ;  and 
one  on  each  side,  the  external  and  internal  recti.     The  eye- 


Fig.  138. 

A,  the  muscles  of  the  right  eyeball  viewed  from  above,  and  B,  of  the  left  eyeball 
viewed  from  the  outer  side;  S.R.,  the  superior  rectus;  ftif.R.,  the  inferior  rectus; 
E.R.,  In.R.,  the  external  rectus;  S.Ob.,  the  superior  oblique;  Iti/.Ob.,  the  inferior 
oblique;  C/i.,  the  chiasmaof  the  optic  nerves  (//.) ;  ///,  the  third  nerve,  which  sup- 
plies all  the  muscles  except  the  superior  oblique  and  the  external  rectus. 


ball  is  completely  imbedded  in  fat  behind  and  laterally  ;  and 
these  muscles  turn  it  as  on  a  cushion  ;  the  superior  rectus 
inclining  the  axis  of  the  eye  upwards,  the  inferior  down- 
wards, the  external  outwards,  the  internal  inwards. 

The  two  oblique  muscles,  upper  and  lower,  are  both 
attached  on  the  outer  side  of  the  ball,  and  rather  behind  its 
centre;  and  they  both  pull  in  a  direction  from  the  point  of 


440  ELEMENTARY   PHYSIOLOGY  less. 

attachment  towards  the  inner  side  of  the  orbit  —  the  lower, 
because  it  arises  here  ;  the  upper,  because,  though  it  arises 
along  with  the  recti  from  the  back  of  the  orbit,  yet,  after  pass- 
ing forwards  and  becoming  tendinous  at  the  upper  and  inner 
corner  of  the  orbit,  it  traverses  a  pulley-like  loop  of  liga- 
ment, and  then  turns  downwards  and  outwards  to  its  inser- 
tion. The  action  of  the  oblique  muscles  is  somewhat 
complicated,  the  upper  rolling  the  eyeball  downwards  and 
outwards,  the  lower  rolling  it  upwards  and  outwards. 

By  means  of  the  contraction  of  these  several  muscles  in 
various  combinations  the  eyeballs  may  be  moved  into  any 
desired  position  and  their  optic  axes  (Fig.  132,  ad)  directed 
straight  towards  any  object.  This  mobility  is  largely  of  use 
in  diminishing  the  necessity  for  such  frequent  movements 
of  the  whole  head  as  would  otherwise  be  necessary.  But 
the  movements  are  also  of  extreme  importance  in  that  they 
bring  the  two  images  of  an  object  upon  corresponding  points 
in  the  retinas  of  the  two  eyes  (see  p.  472)  and  thus  insure 
that  the  object  is  seen  as  single. 

6.  The  Protective  Appendages  of  the  Eye.  —  The  eyelids 
are  folds  of  skin  containing  thin  plates  of  cartilage,  and 
fringed  at  the  edges  with  hairs,  the  eyelashes,  and  with  a 
series  of  small  glands  called  Meibomian,  which  secrete  an 
oily  substance.  Circularly  disposed  fibres  of  striped  muscle 
lie  beneath  the  integuments  of  the  eyelids,  and  constitute 
the  orbicularis  muscle  which  shuts  them  (Fig.  139,  Orb.). 
The  upper  eyelid  is  raised  by  a  special  muscle,  the  levator 
of  the  upper  lid,  which  arises  at  the  back  of  the  orbit 
and  runs  forwards  to  end  in  the  lid.  The  lower  lid  has  no 
special  depressor. 

At  the  edge  of  the  eyelids  the  integument  becomes 
continuous  with  a  delicate,  vascular,  and  highly  nervous, 
membrane,  the  conjunctiva  (Fig.   132,  1'),  which  lines  the 


PROTECTIVE   APPENDAGES   OF  THE  EYE 


441 


■S.U6 


interior  of  the  lids  and  the  front  of  the  eyeball,  its  epithelial 
layer  being  even  continued  over  the  cornea.  The  several 
small  ducts  of  a  gland  which  is  lodged  in  the  orbit,  on  the 
outer  side  of  the  ball  (Fig.  139,  L.G.),  the  lachrymal  gland, 
constantly  pour  its  watery  secretion  into  the  interspace 
between  the  conjunctiva  lining  the  upper  eyelid  and  that 
covering  the  ball.  On  the  nasal  side  of  the  eye  is  a  reddish 
elevation,  between  which  and  the  eyeball  is  a  narrow  vertical 
fold  of  conjunctiva,  the  semilunar  fold  ;  the  latter  is  a  rudi- 
ment of  that  third  eyelid  which  is  to  be  found  in  many 
animals.  Above  and  below,  near  this,  the  edge  of  each  eye- 
lid presents  a  minute  aperture 
(the  punctum  lacrimale),  the 
opening  of  a  small  canal.  The 
canals  from  above  and  below 
converge,  and  open  into  the 
lachrymal  sac  ;  the  upper  blind 
end  of  a  duct  (L.D.,  Fig.  140) 
which  passes  down  from  the 
orbit  to  the  nose,  opening  be- 
low the  inferior  turbinal  bone 
(Fig.  76,  h).     It  is  through  this 

cvatpm    r,f   r-onolc    tVml-    thp   mil       dissected  to  show  Orb.,  the  orbicular 

system  ot  canals  mat  tne  con-   muscle  of  the  eye]ids.  the  pulley  and 

innrtival    mnrnnc;    mpmhnnp    is     insertion  of  the  superior  oblique,  .9. 

juncuvai  mucous   memDrane  ib   oi.   the  inferjor  obUque(  Inf_  obx 
continuous  with  that  of  the  nose  ;   L-G- the  lachr>'mal  s^nd. 
and  it  is  by  them  that  the  secretion  of  the  lachrymal  gland 
is  ordinarily  carried  away  as  fast  as  it  forms. 

But  under  certain  circumstances,  as  when  the  conjunctiva 
is  irritated  by  pungent  vapours,  or  when  painful  emotions 
arise  in  the  mind,  the  secretion  of  the  lachrymal  gland 
exceeds  the  drainage  power  of  the  lachrymal  duct,  and  the 
fluid,  accumulating  between  the  lids,  at  length  overflows  in 
the  form  of  tears. 


Fig.  139. 
The   front   view   of  the  right  eye 


442 


ELEMENTARY   PHYSIOLOGY 


7.  The  Structure  of  the  Retina.  —  If  the  globe  of  the 
eye  be  cut  in  two,  transversely,  so  as  to  divide  it  into  an 
anterior  and  a  posterior  half,  the  retina  will  be  seen  lining 
the  whole  of  the  concave  wall  of  the  posterior  half  as  a 
membrane  of  great  delicacy,  and,  for  the  most  part,  of  even 
texture  and  smooth  surface.  But  almost  exactly  opposite 
the  middle  of  the  posterior  wall,  it  presents  a  slight  oval 
depression  of  a  yellowish  hue,  the  macula  lutea,  or  yellow 
spot  (Fig.  141,  m. I;  Fig.  132,  8"),  —  not  easily  seen,  how- 
ever, unless  the  eye  be  perfectly  fresh,  —  and,  at  some  dis- 
tance from  this,  towards  the  inner  or 
nasal  side  of  the  ball,  is  a  radiating 
appearance,  produced  by  the  en- 
trance of  the  optic  nerve  and  the 
spreading  out  of  its  fibres  into  the 
retina. 

A  very  thin  slice  of  the  retina 
from  its  inner x  to  its  outer  surface, 
in  any  region  except  the  yellow 
spot  and  the  entrance  of  the  optic 
nerve,  may  be  resolved  into  the 
structures  represented  diagram  matically  in  Figs.  142  and 
143.  These  comprise  eight  layers,  seven  of  which  consist 
largely  of  nerve-cells  and  their  processes.  By  the  application 
of  very  special  methods  of  staining  microscopic  sections  the 
true    structure   and  relationships   of  the   layers   have  only 


LG 


CD. 


Fig.  140. 

A  front  view  of  the  left  eye, 
with  the  eyelids  partially  dissected 
to  show  lachrymal  gland,  L.G, 
and  lachrymal  duct,  L.D. 


1  In  the  following  account  of  the  retina,  the  parts  are  described  in  re- 
lation to  the  eyeball.  Thus,  that  surface  of  the  retina  which  touches  the 
vitreous  humour,  and  so  is  nearer  the  centre  of  the  eyeball,  is  called  the 
inner  surface ;  and  that  surface  which  touches  the  choroid  coat  is  called 
the  outer  surface.  And  so  with  the  structures  between  these  two  surfaces; 
that  which  is  called  inner  is  nearer  the  vitreous  humour,  and  that  which  is 
called  outer  is  nearer  the  choroid  coat.  Sometimes  anterior,  or  front,  is 
used  instead  of  inner,  and  posterior  instead  of  outer. 


x  THE   STRUCTURE   OF  THE   RETINA  443 

recently  been  discovered.  Enumerated  from  the  outer 
surface  (in  contact  with  the  choroid)  to  the  inner  surface 
(next  the  vitreous  humour),  these  layers  are  as  follows  :  — 

(i)  The  layer  of  pigment-cells, 
(ii)  The  layer  of  rods  and  cones. 
(hi)  The  outer  nuclear  layer. 
(iv)  The  outer  molecular  layer. 
(v)  The  inner  nuclear  layer, 
(vi)  The  inner  molecular  layer. 
(vii)  The  layer  of  nerve-cells, 
(viii)  The  layer  of  nerve-fibres. 

(i)  When  seen  from  the  surface  by  which  they  are  in 
contact  with  the  choroid,  the  pigment-cells  present  the 
appearance  of  small  black  hexagons  arranged  in  a  sort  of 
mosaic  (Fig.  145,  a).  They  send  long  processes,  loaded 
with  dark  granules,  among  the  rods  and  cones  (3,  c). 

(ii)  The  rods  and  cones  constitute  the  essential  part  of 
the  organ  of  sight,  for  it  is  they  that  receive  the  rays  of  light 
and  inaugurate  the  nervous  impulse.  They  are  processes 
of  modified  epithelial  cells,  and  they  may  also  be  called 
nerve-cells,  since  they  originate  in  the  brain  and  grow  out 
along  the  optic  nerve  to  the  retina.  They  possess  the 
shape,  relative  size,  and  peculiar  striated  appearance  shown 
in  Fig.  143,  and  are  joined  directly  each  with  the  nucleated 
body  of  its  own  cell  lying  in 

(iii)  The  outer  nuclear  layer.  This  layer  receives  its 
name  from  these  nuclei.  The  rod-cells  are  prolonged 
through  this  layer  each  by  a  fine  filament,  which  terminates 
in  the  next  inner  layer  in  a  small  knob.  Each  of  the  cone- 
cells  sends  a  thick  fibre  through  this  layer  to  break  up  into 
a  brush  of  terminal  filaments  in 


444 


ELEM  ENTA RY    PH YSIO LOGY 


(iv)  The  outer  molecular  layer.  Here  the  end-knobs  oi 
the  rod-cells  and  the  terminal  filaments  of  the  cone-cells 
are  in  contiguity,  but  not  in  actual  continuity,  with 
branched  processes  from  a  second  series  of  nerve-cells,  the 
rod  and  cone  bipolar  cells  (Fig.  143,  r.b.p.,  c.b.p).  The 
nucleated  bodies  of  these  bipolar  cells  lie  in 


Fig.    141. 


The  Eyeball   divided   transversely   in    the   Middle  Line  and 
viewed  from  the  front. 


s,  sclerotic;  ch,  choroid,  seen  in  section  only. 

r,  the  cut  edges  of  the  retina;  r.v,  vessels  of  the  retina  springing  from  o,  the 
optic  nerve  or  blind  spot-  ;«./,  the  yellow  spot,  the  darker  spot  in  its  middle  being 
the  fovea  centralis. 


(v)  The  inner  nuclear  layer.  Like  the  rod  and  cone- 
cells  of  the  outer  nuclear  layer  each  sends  inwards  a  fibre 
into  the  next  layer, 

(vi)  The  inner  molecular  layer.  The  cone  bipolar  cells 
here  terminate  in  expanded  branches.  Facing  these,  but 
not  in  actual  continuity  with  them,  lie  the  processes  from 
the  nerve-cells  in  the  so-called  layer  of  nerve-cells.  The 
fibre  from  each  rod  bipolar  cell  passes  through  this  layer 
and  enters 


THE   STRUCTURE   OF  THE    RETINA 


445 


(vii)  The  layer  of  nerve-cells,  to  end  in  branching  pro- 
cesses which  surround  the  body  of  one  of  the  nerve-cells. 
These  nerve-cells,  while  outwardly  in  relation  with  the  rod 
and  cone  bipolar  cells,  on  the  inner  side  are  in  direct  con- 
tinuity each  with  a  fibre  of  the  optic  nerve.     On  their  way 


Outer  surface. 


(i)   Layer  of  pigment-cells, 
(ii)   Layer  of  rods  and  cones. 

(iii)  Outer  nuclear  layer, 
(iv)  Outer  molecular  layer. 

(v)   Inner  nuclear  layer. 

(vi)   Inner  molecular  layer. 

J)(vii)   Layer  of  nerve-cells. 
J:    (viii)   Layer  of  nerve-fibres. 


Inner  surface. 


Fig.  142.  —  Diagrammatic  Section  of  the  Human  Retina  (Schiltze). 
(From  Quain's  Anatomy.) 


over  the  retina  to  the  place  of  exit  of  the  nerve  these  fibres 
form 

(viii)   The  layer  of  nerve-fibres. 

In  this  complex  way  each  rod  and  each  cone  is  brought 


446 


ELEMENTARY   PHYSIOLOGY 


into  relationship  with  a  fibre  of  the  optic  nerve  ;  but,  as  will 
be  readily  understood  from  the  figure,  the  path  of  connec- 
tion in  each  case  shows  two  breaks  in  its  structural  continu- 
ity, and  the  nervous  impulses  originating  in  the  rods  and 


Fig.    143. 


■Diagram   in    Illustration   of  the  Nervous  Structure   of  the 
Retina. 


ii-viii,  the  several  "  layers  "  of  the  retina 

r.o,  r.i,  outer  and  inner  limbs  of  a  rod;  r.f,  rod  fibre;  r.n,  rod  nucleus;  r.b.p,  rod 
bipolar  cell;  c.o,  c.i,  outer  and  inner  limbs  of  a  cone;  c.f,  cone  fibre;  c.n,  cone 
nucleus;  c.b.p.  cone  bipolar  cell:  g.c,  g.c,  two  cells  of  the  nerve-cell  layer;  op.f,  op.f, 
fibres  of  optic  nerve;  s,  h.c,  cells  of  inner  nuclear  lr.yer,  relationships  of  which  are 
not  fully  known. 


THE   STRUCTURE   OE   THE   RETINA 


44? 


cones  must  necessarily  pass  across  these  breaks  on  the  way 
to  the  optic  nerve. 

These    delicate    nervous    structures   are  supported  by  a 
sort  of  framework  of  connective  tissue  of  a  peculiar  kind, 
which  permeates  all  the  layers 
except  the  rods  and  cones  and 
the  pigment-cells. 

The  artery  supplying  the 
retina  with  blood  enters  in  the 
centre  of  the  optic  nerve,  side 
by  side  with  the  outgoing  vein, 
and  then  divides  into  several 
branches  (Fig.  141)  ;  the  re- 
sulting capillaries  exist  simply 
in  the  four  inner  layers. 

In  addition  to  the  bipolar 
cells(Fig.  143,  r.b.p  and  c.b.p), 
which  chiefly  confer  upon  the 
inner  nuclear  layer  the  char- 
acteristic appearance  from 
which  it  derives  its  name, 
other  cells  also  occur  in  this 
layer.  These  are  shown  in 
h.c  and  s ;  but  their  rela- 
tionships to  the  other  struc- 
tural elements  of  the  retina 
are  so  uncertain  that  we  must 
content  ourselves  with  merely 
drawing  attention  to  their  ex- 
istence. 

At  the  entrance  of  the  optic 
nerve  itself,  the  nervous  fibres  predominate,  and  the  rods 
and  cones  are  absent.     In  the  yellow  spot,  on  the  contrary, 


44S 


ELEMENTARY   PHYSIOLOGY 


the  cones  are  abundant- and  close  set,  becoming  at  the  same 
time  longer  and  more  slender,  while  rods  are  scanty,  and 
are  found  only  towards  its  margin.  In  the  centre  of  the 
macula  lutea  (Fig.  144)  the  layer  of  fibres  of  the  optic 
nerve  disappears,  and  all  the  other  layers,  except  that  of 
the  cones,  become  extremely  thin. 


lilP 


Fig.  145.  —  Pigmented  Epithelium  of  the  Human  Retina   (Max  Schultze,  . 
Highly  Magnified. 

a,  cells  seen  from  the  outer  (choroidal)  surface;  i,  two  cells  seen  sidewise,  with 
fine  processes  on  their  inner  side;  c,  a  cell  still  in  connection  with  the  layer  of  rods 
of  the  retina. 


8.  The  Sensation  of  Light.  — The  most  notable  property 
of  the  retina  is  its  power  of  converting  the  vibrations  of  ether, 
which  constitute  the  physical  basis  of  light,  into  a  stimulus 
to  the  fibres  of  the  optic  nerve.  The  central  ends  of  these 
fibres  are  connected  with  certain  parts  of  the  brain  which 
constitute  the  visual  sensorium,  just  as  other  parts,  as  we 
have  seen,  constitute  the  auditory  sensorium.  The  molecu- 
lar disturbances  set  up  in  the  fibres  of  the  optic  nerve  are 
transmitted  to  the  substance  of  the  visual  sensorium,  and 
produce  changes  in  the  latter  giving  rise  to  the  state  of 
feeling  which  we  call  a  sensation  of  light. 

The  sensation  of  light,  it  must  be  understood,  is  the  work 
of  the  visual  sensorium,  not  of  the  retina;  for,  if  certain 
parts  of  the  brain  be  destroyed  or  affected,  no  sensation. 


X  THE   "  BLIND   SPOT"  449 

of  light  is  possible  even  though  the  retina  and  indeed  the 
whole  optic  nerve  be  intact ;  blindness  is  then  the  result, 
because  the  visual  sensorium  cannot  work. 

Light,  falling  directly  on  the  optic  nerve,  does  not  excite 
it ;  the  fibres  of  the  optic  nerve,  in  themselves,  are  as  blind 
as  any  other  part  of  the  body.  But  just  as  the  peculiar  hair- 
cells  of  the  labyrinth,  and  the  organ  of  Corti  of  the  cochlea, 
are  contrivances  for  converting  the  delicate  vibrations  of 
the  endolymph  into  impulses  which  can  excite  the  audi- 
tory nerves,  so  the  structures  in  the  retina  appear  to  be 
adapted  to  convert  the  infinitely  more  delicate  pulses  of 
the  luminiferous  ether  into  stimuli  of  the  fibres  of  the  optic 
nerve. 

9.  The  "Blind  Spot."  — The  sensibility  of  the  different 
parts  of  the  retina  to  light  varies  very  greatly.  The  point 
of  entrance  of  the  optic  nerve  is  absolutely  blind,  as  may 
be  proved  by  a  very  simple  experiment.  Close  the  left 
eye,  and  look  steadily  with  the  right  at  the  cross  on  the 
page,  held  at  ten  or  twelve  inches  distance  from  the  eye. 


* 


The  black  dot  will  be  seen  quite  plainly,  as  well  as  the 
cross.  Now,  move  the  book  slowly  towards  the  eye,  which 
must  be  kept  steadily  fixed  upon  the  cross ;  at  a  certain 
point  the  dot  will  disappear,  but,  as  the  book  is  brought 
still  closer,  it  will  come  into  view  again.  It  results  from 
optical  principles  that,  in  the  first  position  of  the  book,  the 
image  of  the  dot  falls  between  that  of  the  cross  (which 
throughout  lies  upon  the  yellow  spot),  and  the  entrance 
of  the  optic  nerve  :  while,  in  the  second  position,  it  falls  on 

2G 


45° 


ELEMENTARY   PHYSIOLOGY 


the  point  of  entrance  of  the  optic  nerve  itself;  and,  in  the 
third,  it  falls  on  the  other  side  of  that  point.  The  three 
positions  of  the  dot  and  cross,  and  of  the  resulting  images 
of  each  on  the  retina,  are  shown  in  the  accompanying 
figure,  146. 

So  long  as  the  image  of  the  spot  rests  upon  the  entrance 
of  the  optic  nerve,  it  is  not  perceived,  and  hence  this  region 
of  the  retina  is  called  the  blind  spot. 
The  experiment  proves  that  the  vibra- 
tions of  the  ether  are  not  able  to 
excite  the  fibres  of  the  optic  nerve 
itself. 

10.  The  Duration  of  a  Luminous 
Impression. — The  impression  made 
by  light  upon  the  retina  not  only 
remains  during  the  whole  period  of 
the  direct  action  of  the  light,  but  has 
a  certain  duration  of  its  own,  how- 
ever short  the  time  during  which  the 
light  itself  lasts.  A  flash  of  lightning 
is,  practically,  instantaneous,  but  the 
sensation  of  light  produced  by  that 
flash  endures  for  an  appreciable  pe- 
riod. It  is  found,  in  fact,  that  a 
luminous  impression  lasts  for  about 
one-eighth  of  a  second ;  whence  it 
follows,  that  if  any  two  luminous  im- 
pressions are  separated  by  a  less 
interval,  they  are  not  distinguished 
from  one  another. 
For  this  reason  a  "  Catherine-wheel,"  or  a  lighted  stick 
turned  round  very  rapidly  by  the  hand,  appears  as  a  circle 
of  fire ;  and  the  spokes  of  a  coach  wheel  at  speed  are  not 


Fig.  146.  —  Diagram  to  il- 
lustrate the  Blind  Ispot. 


A  SENSATIONS   OF  LIGHT  451 

separately  visible,  but  only  appear  as  a  sort  of  opacity,  01 
film,  within  the  tire  of  the  wheel. 

The  same  fact  is  made  use  of  in  the  production  of  the 
"  animated  photographs  "  which  are  now  so  perfectly  shown 
by  the  apparatus  called  the  kinetoscope.  A  series  of  instan- 
taneous photographs  of  some  object  in  motion,  taken  at  the 
rate  of  many  per  second,  is  printed  on  a  long  transparent 
film  of  celluloid.  The  film  is  then  passed  through  a  magic- 
lantern  at  such  a  rate  that  not  less  than  ten  of  the  consecu- 
tive photographs  are  projected  upon  the  screen  in  each 
second.  At  this  rate,  the  impression  produced  by  one 
photograph  has  not  had  time  to  die  out  before  the  next 
one  produces  its  slightly  different  later  effect.  The  result 
is  that  the  consecutive  pictures  on  the  screen  blend  in  suc- 
cession one  into  the  other  and  so  reproduce  the  appearance 
of  the  original  moving  object. 

11.  Sensations  of  Light  produced  without  the  Action 
of  Light.  —  The  sensation  of  light  may  be  excited  by  other 
causes  than  the  impact  of  the  vibrations  of  the  luminiferous 
ether  upon  the  retina.  Thus,  an  electric  shock  sent  through 
the  eyeball  may  give  rise  to  the  appearance  of  a  flash  of 
light :  and  pressure  on  any  part  of  the  retina  produces  a 
luminous  image,  which  lasts  as  long  as  the  pressure,  and  is 
called  a  phosphene.  If  the  point  of  the  finger  be  pressed 
upon  the  outer  side  of  the  ball  of  the  eye,  the  eyes  being 
shut,  a  luminous  image  —  which,  in  most  cases,  is  dark  in 
the  centre,  with  a  bright  ring  at  the  circumference  (or,  as 
Newton  described  it,  like  the  "  eye  "  in  a  peacock's  tail- 
feather)  —  is  seen ;  and  this  image  lasts  as  long  as  the 
pressure  is  continued.  Most  persons  have  experienced 
the  remarkable  display  of  subjective  fireworks  —  have  seen 
"  stars,"  following  a  heavy  blow  about  the  region  of  the 
eyes. 


4^2  ELEMENTARY   PHYSIOLOGY  less. 

The  sensation  of  light  is,  as  already  explained,  the  work 
of  those  parts  of  the  brain  which,  as  the  visual  sensorium, 
respond  to  the  impulses  reaching  them  through  the  optic 
nerve.  The  retina  is  the  usual  means  of  supplying  the 
impulses  to  the  sensorium  and  may  be  made  to  do  so  by 
light  ordinarily,  but  also  by  other  kinds  of  stimulation. 
But  the  visual  sensorium  itself  may  at  times  be  affected  by 
influences  other  than  those  which  reach  it  from  the  retina. 
In  this  case  also  (subjective)  luminous  sensations  of  the 
most  vivid  and  startling  kind  may  be  experienced,  which 
give  rise  to  delusive  judgments  of  the  most  erroneous  kind 
(see  p.  464). 

12.  The  Functions  of  the  Rods  and  Cones.  —  We  have 
seen  that  the  fibres  of  the  optic  nerve  ramify  in  the  inner 
fourth  of  the  thickness  of  the  retina,  while  the  layer  of  rods 
and  cones  forms  its  outer  fourth.  The  light,  therefore,  must 
fall  first  upon  the  fibres  of  the  optic  nerve,  and  only  after 
traversing  them  and  the  other  layers  of  the  retina  can  it 
reach  the  rods  and  cones.  Consequently,  if  the  fibrillar  of 
the  optic  nerve  themselves  are  capable  of  being  affected  by 
light,  the  rods  and  cones  can  only  be  some  sort  of  supple- 
mentary optical  apparatus.  But,  in  fact,  it  is  the  rods  and 
cones  which  are  affected  by  light,  while  the  fibres  of  the 
optic  nerve  are  themselves  insensible  to  it.  The  evidence 
on  which  this  statement  rests  is  :  — 

(i)  The  blind  spot  is  full  of  nerve-fibres,  but  has  no  cones 
or  rods. 

(ii)  The  yellow  spot,  where  the  most  acute  vision  is  situ- 
ated, is  full  of  close-set  cones,  but  has  no  nerve-fibres. 

(iii)  If  one  goes  into  a  dark  room  with  a  single  small 
bright  candle,  and,  looking  towards  a  dark  wall,  moves  the 
light  up  and  down,  close  to  the  outer  side  of  one  eye,  so  as 
to  allow  the  light  to  fall  very  obliquely  into  the  eye,  what 


x  SENSATIONS   OF  COLOUR  453 

are  called  Furkinje's  figures  are  seen.  These  are  a  series 
of  diverging,  branched,  dark,  sometimes  reddish,  lines  on  an 
illuminated  field.  The  lines  are  the  images  of  shadows 
thrown  by  the  retinal  blood-vessels  (Fig.  141).  As  the 
candle  is  moved  up  and  down,  the  lines  shift  their  position, 
as  shadows  do  when  the  light  which  throws  them  changes 
its  place. 

Now,  as  the  light  falls  on  the  front  face  of  the  retina,  and 
the  images  of  the  vessels  to  which  it  gives  rise  shift  their 
position  as  it  moves,  whatever  constitutes  the  end-organ, 
through  which  light  stimulates  the  fibres  of  the  optic  nerve, 
must  needs  lie  on  the  other  side  of  the  vessels.  But  the 
fibres  of  the  optic  nerve  lie  among  the  vessels,  and  the  only 
nervous  structures  of  the  retina  which  lie  outside  them  are 
the  rods  and  cones  with  their  attached  cell-bodies. 

The  image  of  the  retinal  blood-vessels  may  be  also  very 
readily  seen  by  looking  at  •  a  bright  surface,  such  as  the 
frosted  globe  of  a  burning  lamp  or  a  white  cloud  on  a  sunny 
day,  through  a  pinhole  in  a  card.  When  the  card  is  moved 
rapidly  from  side  to  side,  but  so  as.  to  keep  the  pinhole 
always  within  the  limits  of  the  width  of  the  pupil,  the  retinal 
blood-vessels  are  "seen"  as  a  fine  branched  network  of 
black  lines  in  the  bright  field  of  vision. 

(iv)  Just  as,  in  the  skin,  there  is  a  limit  of  distance  within 
which  two  points  give  only  one  impression,  so  there  is  a 
minimum  distance  by  which  two  points  of  light  falling  on 
the  retina  must  be  separated  in  order  to  appear  as  two. 
And  this  distance  corresponds  pretty  well  with  the  diameter 
of  a  cone. 

13.  Sensations  of  Colour  and  Colour-blindness. — We 
have  spoken  of  the  eye  so  far  simply  as  the  instrument  by 
which  luminous  sensations  arise  when  the  retina  is  stimu- 
lated ;  as  an  instrument  which  enables  us  to  appreciate  the 


454  ELEMENTARY    PHYSIOLOGY  less. 

position  of  a  source  of  light,  and  differences  in  the  intensity 
of  the  light  which  it  emits  or  reflects,  and  hence  to  perceive 
objects  in  the  world  around  us  as  regards  their  position, 
shape,  and  size.  But  the  objects  we  see  are  characterised 
by  something  more  than  mere  shape  and  size ;  they  differ 
also  in  respect  of  what  we  call  their  colour. 

When  we  look  at  a  rainbow  we  are  conscious  of  seven 
broadly  distinct  kinds  of  colour-sensations  ;  these  are  red, 
orange,  yellow,  green,  blue,  indigo-blue,  and  violet,  and 
when  ordinary  white  light  is  passed  through  a  prism  and 
then  allowed  to  fall  into  the  eye  we  experience  the  same 
seven  coloured  sensations.  The  prism  has,  in  fact,  resolved 
the  light  into  its  several  coloured  constituents,  and  these  are 
known  as  the  "  colours  of  the  spectrum."  Each  colour 
which  we  recognise  as  such  is  characterised,  just  as  in  the 
case  of  sounds,  by  certain  qualities;  these  are  (i)  hue,  or 
colour,  as  we  ordinarily  use  the  word  to  denote  what  we  call 
reds,  greens,  blues,  and  so  on.  This  quality  is  dependent 
on  the  wave-length  of  the  ethereal  vibrations  which  are  giv- 
ing rise  to  the  sensation,  and  hence  corresponds  to  the 
"  pitch "  of  a  sound,  (ii)  intensity  or  brightness.  This 
depends  on  the  amount  of  light  which  falls  on  the  retina  in 
a  given  time  and  corresponds  to  the  loudness  of  a  sound, 
(iii)  saturation,  or  the  amount  of  admixture  with  white 
light.  Thus,  we  speak  of  a  colour  as  being  "  pale  "  if 
mixed  with  much  white,  and  as  being  "deep,"  "rich,"  or 
"full,"  if  highly  saturated,  i.e.  unmixed  with  white. 

The  colours  of  objects  depend  on  the  power  they  possess 
of  absorbing  some  of  the  constituents  of  ordinary  white  light 
and  allowing  others  to  pass  or  to  be  reflected.  Thus,  a 
piece  of  transparent  glass  is  red  if  it  allows  the  red  rays  to 
pass  through  to  the  eye  and  stops  the  others.  Similarly,  the 
colour  of  an  opaque  red  object  is  due  to  an  absorption  of  the 


X  SENSATIONS   OF  COLOUR  455 

spectral  colours  other  than  red,  and  the  reflection  of  the 
unabsorbed  red  rays. 

When  white  light  has  been  split  up  into  its  coloured  con- 
stituents by  means  of  a  prism,  these  may  be  gathered  up 
again  by  a  second  prism,  suitably  placed,  and  recombined 
to  make  white  light.  In  this  experiment  the  several  col- 
ours of  the  spectrum  are  mixed  once  more  after  having  been 
sorted  out  or  separated,  and  the  mixing  is  a  physical  process. 
But  colours  may  also  be  mixed  physiologically  by  taking  ad- 
vantage of  that  persistence  of  luminous  impressions  to  which 
we  have  already  drawn  attention  (p.  450).  Thus,  if  the 
several  colours  of  the  spectrum  are  painted  in  sectors  on  a 
circular  disc  and  the  disc  is  made  to  spin  rapidly  round  its 
centre,  the  sensations  due  to  each  colour  are  blended  to- 
gether and  the  disc  appears  white.  The  common  instru- 
ment used  in  this  mode  of  mixing  colours  is  called  a  "  colour 
lop." 

By  the  use  of  a  colour  top  it  is  at  once  possible  to  mix 
not  merely  all  the  spectral  colours  but  any  two  or  three  of 
them.  Experimenting  in  this  way  with  pairs  of  colours  we 
find  that  there  are  several  pairs  which  when  mixed  give  rise 
to  the  sensation  of  white  :  thus  red  and  green,  orange  and 
blue,  yellow  and  indigo-blue,  greenish-yellow  and  violet. 
Colours  which  when  mixed  in  this  way  in  pairs  give  white 
are  known  as  complementary  colours,  and  every  colour  has 
some  other  colour  which  is  complementary  to  it.  If  instead 
of  mixing  the  colours  in  pairs  we  mix  them  in  threes,  then 
it  becomes  still  more  easy  to  produce  a  resultant  white. 
Thus,  by  mixing  red,  green,  and  blue,  with  due  regard  to 
the  relative  amount  and  intensity  of  each,  an  excellent  white 
is  readily  obtained.  But  these  three  colours  enable  us  to 
do  more  than  merely  produce  white.  By  properly  adjust- 
ing the  proportions  of  each  on  the  disc  of  the  colour  top  we 


456  ELEMENTARY   PHYSIOLOGY  less 

can  easily  produce  an  orange  and  a  yellow,  as  also  a  violet, 
In  other  words,  these  three  colours  and  their  mixtures  give 
rise  to  all  the  several  kinds  of  colour-sensation  which  we 
derive  from  a  spectrum.  Further,  by  suitable  mixture  of 
these  colours,  together  with  white  or  black,  we  can  pro- 
duce the  other  colours  which  we  see  in  natural  objects 
around  us  but  which  are  wanting  in  the  spectrum.  Thus, 
purple  is  extremely  common  in  the  world  and  can  be  made 
at  once  by  mixing  red  and  blue.  Hence  these  three  col- 
ours have  come  to  be  regarded  as  primary  colours,  and  we 
may  speak  of  the  sensations  to  which  they  give  rise  as 
primary  sensations. 

The  foregoing  considerations  lead  at  once  to  the  view 
that  all  our  sensations  of  colour  maybe  regarded  as  the  out- 
come of  a  very  limited  number  (three)  of  simple  or  primary 
sensations,  corresponding  to  red,  green,  and  blue.  In 
accordance  with  this  fact  a  theory  has  been  put  forward1 
that  there  are  in  the  nervous  apparatus  of  vision  three  kinds 
of  nervous  structure,  the  nature  and  exact  location  of  which 
are  wholly  unknown,  but  of  which  each  corresponds  to  one 
of  the  primary  colours  and  is  most  easily  set  in  action  by 
one  of  these  colours.  Thus,  the  stimulation  of  one  of  them 
gives  rise  to  one  of  the  primary  sensations,  the  simultaneous 
stimulation  of  all  three  to  the  same  extent  gives  rise  to  the 
sensation  of  white,  and  their  simultaneous  stimulation  to 
varying  degrees  gives  rise  to  all  the  other  sensations  of  col- 
our of  which  we  are  at  any  time  conscious. 

This  theory  attempts  to  account  for  observed  facts,  and 
goes  a  long  way  in  doing  so  ;  but  it  does  not  account  com- 

1  This  theory  was  first  propounded  by  an  Englishman,  Dr.  Thomas 
Young,  the  originator  of  the  undulatory  theory  of  light.  In  later  times  it 
was  adopted  and  amplified  by  Helmholtz,  and  is  therefore  known  as  the 
Young-Helmholtz  theory. 


x  COLOUR-BLINDNESS  457 

pletely  for  all  the  facts  of  colour  vision.  Other  theories, 
likewise  insufficient,  have  been  proposed,  but  a  discussion 
of  their  respective  merits,  to  be  of  value,  must  necessarily  be 
lengthy  and  detailed,  and  hence  out  of  place  in  an  element- 
ary text-book. 

The  excitability  of  the  retina  is  readily  exhausted.  Thus, 
looking  at  a  bright  light  rapidly  renders  the  part  of  the 
retina  on  which  the  light  falls  insensible  ;  and  on  looking 
from  the  bright  light  towards  a  moderately  lighted  surface, 
a  dark  spot,  arising  from  a  temporary  blindness  of  the  retina 
in  this  part,  appears  in  the  field  of  view.  If  the  bright  light 
be  of  one  colour,  the  part  of  the  retina  on  which  it  falls  be- 
comes insensible  to  the  rays  of  that  colour,  but  not  to  the 
other  rays  of  the  spectrum.  This  is  the  explanation  of  the 
appearance  of  what  are  called  negative  after-images.  For 
example,  if,  as  in  the  form  in  which  the  experiment  is  most 
commonly  made,  a  bright  red  wafer  be  stuck  upon  a  sheet 
of  white  paper,  and  steadily  looked  at  for  some  time  with 
one  eye,  when  the  eye  is  turned  aside  to  the  white  paper  a 
greenish  spot  will  appear,  of  about  the  size  and  shape  of  the 
wafer.  The  red  image  has,  in  fact,  fatigued  the  part  of  the 
retina  on  which  it  fell  for  red  light,  but  has  left  it  sensitive 
to  the  remaining  coloured  rays  of  which  white  light  is  com- 
posed. But  we  know  that,  if  from  the  variously  coloured 
rays  which  make  up  the  spectrum  of  white  light  we  take 
away  all  the  red  rays,  the  remaining  rays  together  make  up 
a  sort  of  green.  So  that,  when  white  light  falls  upon  this 
part,  the  red  rays  in  the  white  light  having  no  effect,  the 
result  of  the  operation  of  the  others  is  a  greenish  hue. 
The  colour  of  the  negative  after-image  is  thus  of  necessity 
complementary  to  that  of  the  object  looked  at.  If  the  wafer 
be  green,  the  after-image  is  of  course  red. 

Colour-blindness.  —  Most  people  agree  very  closely  as  to 


458  ELEMENTARY   PHYSIOLOGY  less,  x 

differences  between  different  colours  and  different  parts  of 
the  spectrum.  But  there  are  exceptions.  Thus  a  certain 
number  of  persons  see  very  little  difference  between  the 
colour  which  most  people  call  red,  and  that  which  most  peo- 
ple call  green.  Such  colour-blind  persons  are  perhaps  unable 
to  distinguish  between  the  leaves  of  a  cherry-tree  and  its  fruit 
by  the  colour  of  the  two  ;  they  are  only  aware  of  a  difference 
of  shape  between  the  two.  Cases  of  this  "red-blindness" 
or  " red-green  "  blindness  are  not  uncommon;  but  other 
forms  of  colour-blindness  are  much  more  rare  ;  and  extremely 
rare,  though  of  undoubted  occurrence,  are  the  cases  of  those 
who  are  wholly  colour-blind,  i.e.  to  whom  all  colours  are  mere 
shades  of  one  tint. 

This  peculiarity  of  colour-blindness  is  simply  unfortunate 
for  most  people,  but  it  may  be  dangerous  if  unknowingly 
possessed  by  engine-drivers  or  sailors,  particularly  since  red- 
green  colour-blindness  is  most  common  and  red  and  green 
are  exactly  the  two  colours  ordinarily  used  for  signals.  It 
probably  arises  either  from  a  defect  in  the  retina,  which 
renders  that  organ  unable  to  respond  to  different  kinds  of 
luminous  vibrations,  and  consequently  insensible  to  red,  yel- 
low, or  other  rays,  as  the  case  may  be ;  or  the  fault  may  lie 
in  the  visual  sensorium  itself. 

For  ordinary  purposes  colour  perception  may  be  most 
easily  and  successfully  tested  by  asking  the  person  under 
examination  to  make  matches  between  skeins  of  coloured 
wool.  In  this  way  it  is  found  that  a  red-green  colour-blind 
person  matches  a  red  with  a  green  skein. 

The  phenomena  of  colour-blindness  can,  to  a  certain 
extent  at  least,  be  explained  according  to  the  theory  of  col- 
our-vision which  has  been  given  above.  Thus,  a  red-green 
colour-blind  person  is  supposed  to  lack  either  the  red-per- 
ceiving or  the  green-perceiving  structures  normally  present 
either  in  the  retina  or  the  visual  sensorium. 


LESSON    XI 

THE  COALESCENCE  OF  SENSATIONS  WITH  ONE 
ANOTHER  AND  WITH  OTHER  STATES  OF  CON- 
SCIOUSNESS 

1.  Sensations  may  be  Simple  or  Composite.  —  In  ex- 
plaining the  functions  of  the  sensory  organs,  we  have 
hitherto  confined  ourselves  to  describing  the  means  by 
which  the  physical  agent  of  a  sensation  is  enabled  to  irritate 
a  given  sensory  nerve ;  and  to  giving  some  account  of  the 
simple  sensations  which  are  thus  evolved. 

Simple  sensations  of  this  kind  are  such  as  might  be  pro- 
duced by  the  irritation  of  a  single  nerve-fibre,  or  of  several 
nerve-fibres  by  the  same  agent.  Such  are  the  sensations  of 
contact,  of  warmth,  of  sweetness,  of  an  odour,  of  a  musical 
note,  of  whiteness,  or  redness. 

But  very  few  of  our  sensations  are  thus  simple.  Most  of 
even  those  which  we  are  in  the  habit  of  regarding  as  simple, 
are  really  compounds  of  different  simultaneous  sensations, 
or  of  present  sensations  with  past  sensations,  or  with  those 
feelings  of  relation  which  form  the  basis  of  judgments.  For 
example,  in  the  preceding  cases  it  is  very  difficult  to  sepa- 
rate the  sensation  of  contact  from  the  judgment  that  some- 
thing is  touching  us  ;  of  sweetness,  from  the  idea  that  some- 
thing is  in  the  mouth  ;  of  sound  or  light,  from  the  judgment 
that  something  outside  us  is  shining  or  sounding. 

The  sensations  of  smell  are  those  which  are  least  compli 
459 


460  ELEMENTARY   PHYSIOLOGY  less. 

cated  by  accessories  of  this  sort.  Thus,  particles  of  musk 
diffuse  themselves  with  great  rapidity  through  the  nasal  pas- 
sages and  give  rise  to  the  sensation  of  a  powerful  odour. 
But  beyond  a  broad  notion  that  the  odour  is  in  the  nose, 
this  sensation  is  unaccompanied  by  any  ideas  of  locality  and 
direction.  Still  less  does  it  give  rise  to  any  conception  of 
form,  or  size,  or  force,  or  of  succession,  or  contemporaneity. 
If  a  man  had  no  other  sense  than  that  of  smell,  and  musk 
were  the  only  odorous  body,  he  could  have  no  sense  of 
outness  —  no  power  of  distinguishing  between  the  external 
world  and  himself. 

Contrast  this  with  what  may  seem  to  be  the  equally 
simple  sensation  obtained  by  drawing  the  finger  along  the 
table,  the  eyes  being  shut.  This  act  gives  one  the  sensation 
of  a  flat,  hard  surface  outside  one's  self,  which  sensation 
appears  to  be  just  as  simple  as  the  odour  of  musk,  but 
is  really  a  complex  state  of  feeling  compounded  of — 

(a)    Pure  sensations  of  contact. 

(&)  Pure  muscular  sensations  of  two  kinds,  —  the  one 
arising  from  the  resistance  of  the  table,  the  other  from  the 
actions  of  those  muscles  which  draw  the  finger  along. 

(<r)  Ideas  of  the  order  in  which  these  pure  sensations 
succeed  one  another. 

(d)  Comparisons  of  these  sensations  and  their  order, 
with  the  recollection  of  like  sensations  similarly  arranged, 
which  have  been  obtained  on  previous  occasions. 

(<?)  Recollections  of  the  impressions  of  extension,  flat- 
ness, etc.,  made  on  the  organ  of  vision  when  these  previous 
tactile  and  muscular  sensations  were  obtained. 

Thus,  in  this  case,  the  only  pure  sensations  are  those 
of  contact  and  muscular  action.  The  greater  part  of  what 
we  call  the  sensation  is  a  complex  mass  of  present  and 
recollected  sensations  and  judgments. 


xi  SENSATIONS  461 

Should  any  doubt  remain  that  we  do  thus  mix  up  our 
sensations  with  our  judgments  into  one  indistinguishable 
whole,  shut  the  eyes  as  before,  and,  instead  of  touching  the 
table  with  the  finger,  take  a  round  lead  pencil  between 
the  fingers,  and  draw  that  along  the  table.  The  "  sensation  " 
of  a  flat  hard  surface  will  be  just  as  clear  as  before ;  and 
yet  all  that  we  touch  is  the  round  surface  of  the  pencil,  and 
the  only  pure  sensations  we  owe  to  the  table  are  those  afforded 
by  the  muscular  sense.  In  fact,  in  this  case,  our  "  sensation  " 
of  a  flat  hard  surface  is  entirely  a  judgment  based  upon  what 
the  muscular  sense  tells  us  is  going  on  in  certain  muscles. 

A  still  more  striking  case  of  the  tenacity  with  which 
we  adhere  to  complex  judgments,  which  we  conceive  to 
be  pure  sensations,  and  are  unable  to  analyse  otherwise 
than  by  a  process  of  abstract  reasoning,  is  afforded  by  our 
sense  of  roundness. 

Any  one  taking  a  marble  between  two  fingers  will  say 
that  he  feels  it  to  be  a  single  round  body ;  and  he  will 
probably  be  as  much  at  a  loss  to  answer  the  question  how 
he  knows  that  it  is  round,  as  he  would  be  if  he  were  asked 
how  he  knows  that  a  scent  is  a  scent. 

Nevertheless,  this  notion  of  the  roundness  of  the  marble 
is  really  a  very  complex  judgment,  and  that  it  is  so  may  be 
shown  by  a  simple  experiment.  If  the  index  and  middle 
fingers  be  crossed,  and  the  marble  placed  between  them, 
so  as  to  be  in  contact  with  both,  it  is  almost  impossible  to 
avoid  the  belief  that  there  are  two  marbles  instead  of 
one.  Even  looking  at  the  marble  and  seeing  that  there 
is  only  one,  does  not  weaken  the  apparent  proof  derived 
from  touch  that  there  are  two.1 

1  A  ludicrous  form  of  this  experiment  is  to  apply  the  crossed  fingers  to 
the  end  of  the  nose,  when  it  at  once  appears  double ;  and,  in  spite  of  the 
absurdity  of  the  conviction,  the  mind  cannot  expel  it  so  long  as  the  sensa- 
tions last. 


4b2  ELEMENTARY   PHYSIOLOGY  less. 

The  fact  is,  that  our  notions  of  singleness  and  round- 
ness are,  really,  highly  complex  judgments  based  upon 
a  few  simple  sensations  ;  and  when  the  ordinary  conditions 
of  those  judgments  are  reversed,  the  judgment  is  also 
reversed. 

With  the  index  and  the  middle  fingers  in  their  ordinary 
position,  it  is  of  course  impossible  that  the  outer  sides 
of  each  should  touch  opposite  surfaces  of  one  spheroidal 
body.  If,  in  the  natural  and  usual  position  of  the  fingers, 
their  outer  surfaces  simultaneously  give  us  the  impression 
of  a  spheroid  (which  itself  is  a  complex  judgment),  it  is 
in  the  nature  of  things  that  there  must  be  two  spheroids. 
But,  when  the  fingers  are  crossed  over  the  marble,  the  outer 
side  of  each  finger  is  really  in  contact  with  a  spheroid ; 
and  the  mind,  taking  no  cognizance  of  the  crossing,  judges 
in  accordance  with  its  universal  experience,  that  two 
spheroids,  and  not  one  give  rise  to  the  sensations  which 
are  perceived. 

2.  Judgments,  not  Sensations,  are  Delusive.  —  Phenom- 
ena of  the  kind  described  in  the  preceding  section  are 
not  uncommonly  called  delusions  of  the  senses ;  but  there 
is  no  such  thing  as  a  fictitious,  or  delusive  sensation.  A 
sensation  must  exist  to  be  a  sensation,  and  if  it  exists,  it  is 
real  and  not  delusive.  But  the  judgments  we  form  respect- 
ing the  causes  and  conditions  of  the  sensations  of  which  we 
are  aware,  are  very  often  erroneous  and  delusive  enough; 
and  such  judgments  may  be  brought  about  in  the  domain 
of  every  sense,  either  by  artificial  combinations  of  sensations, 
or  by  the  influence  of  unusual  conditions  of  the  body  itself. 
The  latter  give  rise  to  what  are  called  subjective  sensations. 

Mankind  would  be  subject  to  fewer  delusions  than  they 
are,  if  they  constantly  bore  in  mind  their  liability  to  false 
judgments   due    to   unusual    combinations,    either    artificial 


xi  SUBJECTIVE   SENSATIONS  463 

or  natural,  of  true  sensations.  Men  say,  "  I  felt,"  "  I 
heard,"  "  I  saw  "  such  and  such  a  thing,  when,  in  ninety- 
nine  cases  out  of  a  hundred,  what  they  really  mean  is, 
that  they  judge  that  certain  sensations  of  touch,  hearing, 
or  sight,  of  which  they  are  conscious,  were  caused  by  such 
and  such  things. 

3.  Subjective  Sensations.  —  Among  subjective  sensations 
within  the  domain  of  touch,  are  the  feelings  of  creeping 
and  the  prickling  of  the  skin,  which  may  sometimes  be 
due  to  certain  states  of  the  circulation,  but  probably  more 
frequently  to  processes  going  on  in  the  central  nervous 
system.  The  subjective  evil  smells  and  bad  tastes  which 
accompany  some  diseases  are,  in  a  similar  way,  very  proba- 
bly due  to  disturbances  in  the  brain  in  the  central  end- 
organs  of  the  nerves  of  smell  and  taste. 

Many  persons  are  liable  to  what  may  be  called  auditory 
spectra  —  music  of  various  degrees  of  complexity  sounding 
in  their  ears,  without  any  external  cause,  while  they  are 
wide  awake.  I  know  not  if  other  persons  are  similarly 
troubled,  but  in  reading  books  written  by  persons  with 
whom  I  am  acquainted,  I  am  sometimes  tormented  by 
hearing  the  words  pronounced  in  the  exact  way  in  which 
these  persons  would  utter  them,  any  trick  or  peculiarity 
of  voice  or  gesture  being  also  very  accurately  reproduced. 
And  I  suppose  that  every  one  must  have  been  startled,  at 
times,  by  the  extreme  distinctness  with  which  his  thoughts 
have  embodied  themselves  in  apparent  voices. 

The  most  wonderful  exemplifications  of  subjective  sensa- 
tion, however,  are  afforded  by  the  organ  of  sight. 

Any  one  who  has  witnessed  the  sufferings  of  a  man 
labouring  under  delirium  tremens  (a  disease  produced  by 
excessive  drinking),  from  the  marvellous  distinctness  of  his 
visions,  which  sometimes  take  the  forms  of  devils,  sometimes 


464  ELEMENTARY   PHYSIOLOGY  less. 

of  creeping  animals,  but  almost  always  of  something  fearful 
or  loathsome,  will  not  doubt  the  intensity  of  subjective 
sensations  in  the  domain  of  vision. 

But  in  order  that  delusive  visions  of  great  distinctness 
should  appear,  it  is  not  necessary  for  the  nervous  system 
to  be  thus  obviously  deranged.  People  in  the  full  posses- 
sion of  their  faculties,  and  of  high  intelligence,  may  be 
subject  to  such  appearances,  for  which  no  distinct  cause 
can  be  assigned.  An  excellent  illustration  of  this  is  the 
famous  case  of  Mrs.  A.  given  by  Sir  David  Brewster  in  his 
Natural  Magic.  This  lady  was  subject  to  unusually  vivid 
auditory  and  ocular  spectra.  Thus,  on  one  occasion  she 
saw  her  husband  standing  before  her,  and  looking  fixedly  at 
her  with  a  serious  expression,  though  at  the  time  he  was  at 
another  place.  On  another  occasion  she  heard  him  repeat- 
edly call  her,  though  at  the  time  he  was  not  anywhere  near. 
On  another  occasion  she  saw  a  cat  in  the  room  lying  on  the 
rug ;  and  so  vivid  was  the  illusion  that  she  had  great  diffi- 
culty in  satisfying  herself  that  really  there  was  no  cat  there. 
The  whole  account  is  well  worthy  of  perusal. 

It  is  obvious  that  nothing  but  the  singular  courage  and 
clear  intellect  of  Mrs.  A.  prevented  her  from  becoming  a 
mine  of  ghost  stories  of  the  most  excellently  authenticated 
kind.  And  the  particular  value  of  her  history  lies  in  its 
showing  that  the  clearest  testimony  of  the  most  unimpeach- 
able witness  may  be  quite  inconclusive  as  to  the  objective 
reality  of  something  which  the  witness  has  seen. 

Mrs.  A.  undoubtedly  saw  what  she  said  she  saw.  The 
evidence  of  her  eyes  as  to  the  existence  of  the  apparitions, 
and  of  her  ears  to  those  of  the  voices,  was,  in  itself,  as 
perfectly  trustworthy  as  their  evidence  would  have  been 
had  the  objects  t  really  existed.  For  there  can  be  no 
doubt  that  exactly  those  parts  of  her  retina  which  would 


xi  DELUSIONS   OF  JUDGMENT  465 

have  been  affected  by  the  image  of  a  cat,  and  those  parts 
of  her  auditory  organ  which  would  have  been  set  vibrating 
by  her  husband's  voice,  or  rather  the  portions  of  the 
sensorium  with  which  those  organs  of  sense  are  connected, 
were  thrown  into  a  corresponding  state  of  activity  by  some 
internal  cause. 

What  the  senses  testify  is  neither  more  nor  less  than  the 
fact  of  their  own  affection.  As  to  the  cause  of  that  affec- 
tion they  really  say  nothing,  but  leave  the  mind  to  form  its 
own  judgment  on  the  matter.  A  hasty  or  superstitious  per- 
son in  Mrs.  A.'s  place  would  have  formed  a  wrong  judgment, 
and  would  have  stood  by  it  on  the  plea  that  "  she  must 
believe  her  senses." 

4.  Delusions  of  Judgment.  —  The  delusions  of  the  judg- 
ment, produced  not  by  abnormal  conditions  of  the  body, 
but  by  unusual  or  artificial  combinations  of  sensations,  or  by 
suggestions  of  ideas,  are  exceedingly  numerous,  and  occa- 
sionally are  not  a  little  remarkable. 

Some  of  those  which  arise  out  of  the  sensation  of  touch 
have  already  been  noted.  We  do  not  know  of  any  produced 
through  smell  or  taste,  but  hearing  is  a  fertile  source  of  such 
errors. 

What  is  called  ventriloquism  (speaking  from  the  belly), 
and  is  not  uncommonly  ascribed  to  a  mysterious  power  of 
producing  voice  somewhere  else  than  in  the  larynx,-  depends 
entirely  upon  the  accuracy  with  which  the  performer  can 
simulate  sounds  of  a  particular  character,  and  upon  the  skill 
with  which  he  can  suggest  a  belief  in  the  existence  of  the 
causes  of  these  sounds.  Thus,  if  the  ventriloquist  desire  to 
create  the  belief  that  a  voice  issues  from  the  bowels  of  the 
earth,  he  imitates  with  great  accuracy  the  tones  of  such  a 
half-stifled  voice,  and  suggests  the  existence  of  some  one 
uttering  it  by  directing  his  answers  and  gestures  towards  the 

2H 


466  ELEMENTARY   PHYSIOLOGY  less. 

ground.  These  gestures  and  tones  are  such  as  would  be 
produced  by  a  given  cause ;  and,  no  other  cause  being 
apparent,  the  mind  of  the  bystander  insensibly  judges  the 
suggested  cause  to  exist. 

The  delusions  of  the  judgment  through  the  sense  of  sight 
—  optical  delusions,  as  they  are  called  —  are  more  numer- 
ous than  any  others,  because  such  a  great  number  of  what 
we  think  to  be  simple  visual  sensations  are  really  very 
complex  aggregates  of  visual  sensations,  tactile  sensations, 
judgments,  and  recollections  of  former  sensations  and 
judgments. 

It  will  be  instructive  to  analyse  some  of  these  judg- 
ments into  their  principles,  and  to  explain  the  delusions  by 
the  application  of  these  principles. 

5.  The  Inversion  of  the  Visual  Image.  —  When  we  look 
at  an  external  object,  the  image  of  the  object  falls  on  the 
retina  at  the  end  of  the  visual  axis,  i.e.  a  line  joining  the 
object  and  the  retina  and  traversing  a  particular  region  of 
the  centre  of  the  eye.  Conversely,  when  a  part  of  the  retina 
is  excited,  by  whatever  means,  the  sensation  is  referred  by 
the  mind  to  some  cause  outside  the  body  in  the  direction  of 
the  visual  axis. 

When  we  look  at  an  external  object  which  is  felt  by  the 
touch  to  be  in  a  given  place,  the  image  of  the  object  falls 
upon  a  certain  part  of  the  retina.  Conversely,  ivhen  a  part 
of  the  retina  is  excited,  by  whatever  means,  the  sensation  is 
referred  bv  the  mind  to  some  cause  outside  the  body  occupy- 
ing such  a  position  that  its  image  would  fall  on  that  part. 

It  is  for  this  reason  that  when  a  phosphene  is  created  by 
pressure,  say  on  the  outer  and  lower  side  of  the  eyeball,  the 
luminous  image  appears  to  lie  above,  and  to  the  inner  side 
of,  the  eye.  Any  external  object  which  could  produce  the 
sense  of  light  in  the  part  of  the  retina  pressed  upon  must, 


xi  EVERY  IMAGE  REFERRED  TO   AN   OBJECT  467 

owing  to  the  inversion  of  the  retinal  images  (see  p.  433).. 
in  fact  occupy  this  position  ;  and  hence  the  mind  refers  the 
light  seen  to  an  object  in  that  position. 

The  same  kind  of  explanation  is  applicable  to  the  appar- 
ent paradox  that,  while  all  the  pictures  of  external  objects 
are  certainly  inverted  on  the  retina  by  the  refracting  media 
of  the  eye,  we  nevertheless  see  them  upright.  It  is  difficult 
to  understand  this,  until  one  reflects  that  the  retina  has,  in 
itself,  no  means  of  indicating  to  the  mind  which  of  its  parts 
lies  at  the  top,  and  which  at  the  bottom  ;  and  that  the 
mind  learns  to  call  an  impression  on  the  retina  high  or  low, 
right  or  left,  simply  on  account  of  the  association  of  such  an 
impression  with  certain  coincident  tactile  impressions.  In 
other  words,  when  one  part  of  the  retina  is  affected,  the 
object  causing  the  affection  is  found  to  be  near  the  right 
hand  ;  when  another,  the  left ;  when  another,  the  hand  has 
to  be  raised  to  reach  the  object ;  when  yet  another,  it  has 
to  be  depressed  to  reach  it.  And  thus  the  several  impres- 
sions on  the  retina  are  called  right,  left,  upper,  lower,  quite 
irrespectively  of  their  real  positions,  of  which  the  mind  has, 
and  can  have,  no  cognizance. 

6.  Every  Image  referred  to  an  Object.  —  When  an 
external  body  is  ascertained  by  touch  to  be  single,  it  forms 
but  one  image  on  the  retina  of  a  single  eye;  and  when  two  or 
more  images  fall  on  the  retina  of  a  single  eye,  they  ordinarily 
proceed  from  a  corresponding  number  of  bodies  which  are 
distinct  to  the  touch. 

Conversely,  the  sensation  of  tivo  or  more  images  is  judged 
by  the  mind  to  proceed  from  two  or  mo?-e  objects. 

If  two  pin-holes  be  made  in  a  piece  of  cardboard  at  a 
distance  less  than  the  diameter  of  the  pupil,  and  a  small 
object  like  the  head  of  a  pin  be  held  pretty  close  to  the 
eye,  and  viewed   through  these   holes,  two  images  of  the 


458  ELEMENTARY   PHYSIOLOGY  less. 

head  of  the  pin  will  be  seen.  The  reason  of  this  is,  that  the 
rays  of  light  from  the  head  of  the  pin  are  split  by  the  card 
into  two  minute  pencils,  which  pass  into  the  eye  on  either 
side  of  its  centre,  and,  on  account  of  the  nearness  of  the 
pin  to  the  eye,  meet  the  retina  before  they  can  be  united 
again  and  brought  to  one  focus.  Hence  they  fall  on  differ- 
ent parts  of  the  retina,  and  each  pencil  of  rays  being  very 
small  makes  a  tolerably  distinct  image  of  its  own  of  the 
pin's  head  on  the  retina.  Each  of  these  images  is  now 
referred  outward  (p.  466)  and  two  pins  are  apparently  seen 
instead  of  one.  A  like  explanation  applies  to  multiplying 
glasses  and  doubly  refracting  crystals,  both  of  which,  in  their 
own  ways,  split  the  pencils  of  light  proceeding  from  a  single 
object  into  two  or  more  separate  bundles.  These  give  rise 
to  as  many  images,  each  of  which  is  referred  by  the  mind 
to  a  distinct  external  object. 

7.  The  Judgment  of  Distance  and  Size  by  the  Bright- 
ness and  Size  of  Visual  Images.  —  Certain  visual  phenom- 
ena  ordinarily  accompany  those  prodiicts  of  tactile  sensation 
to  which  we  give  the  na7?ie  of  size,  distance,  and  form. 
Thus,  other  things  being  alike,  the  space  of  the  retina 
covered  by  the  image  of  a  large  object  is  larger  than  that 
covered  by  a  small  object:  while  that  covered  by  an  object 
when  near  is  larger  than  that  covered  by  the  same  object 
when  distant ;  and,  other  conditions  being  alike,  a  near 
object  is  more  brilliant  than  a  distant  one.  Furthermore, 
the  shadows  of  objects  differ  according  to  the  forms  of  their 
surfaces,  as  determined  by  touch. 

Conversely,  if  these  visual  sensations  can  be  produced,  they 
inevitably'  suggest  a  belief  in  the  existence  of  objects  competent 
to  produce  the  corresponding  tactile  sensations. 

What  is  called  perspective,  whether  solid  or  aerial,  in 
drawing,  or  painting,  depends  on  the  application  of  these 


XI  JUDGMENT   OF   DISTANCE   AND    SIZE  469 

principles.  It  is  a  kind  of  visual  ventriloquism  —  the 
painter  putting  upon  his  canvas  all  the  conditions  requisite 
for  the  production  of  images  on  the  retina  having  the  size, 
relative  form,  and  intensity  of  colour  of  those  which  would 
actually  be  produced  by  the  objects  themselves  in  nature. 
And  the  success  of  his  picture,  as  an  imitation,  depends 
upon  the  closeness  of  the  resemblance  between  the  images 
it  produces  on  the  retina  and  those  which  would  be  pro- 
duced by  the  objects  represented. 

To  most  persons  the  image  of  a  pin,  at  three  or  four 
inches  from  the  eye,  appears  blurred  and  indistinct  —  the 
eye  not  being  capable  of  adjustment  to  so  short  a  focus.  If 
a  small  hole  be  made  in  a  piece  of  card,  the  circumferential 
rays  which  cause  the  blur  are  cut  off,  and  the  image  becomes 
distinct.  But  at  the  same  time  it  is  magnified,  or  looks 
bigger,  because  the  image  of  the  pin,  in  spite  of  the  loss  of 
the  circumferential  rays,  occupies  a  much  larger  extent  of 
the  retina  when  close  than  when  distant.  All  convex  glasses 
produce  the  same  effect  —  while  concave  lenses  diminish 
the  apparent  size  of  an  object,  because  they  diminish  the 
size  of  its  image  on  the  retina. 

As  is  well  known,  objects  appear  larger  when  seen  in  a 
fog.  In  this  case  the  actual  size  of  the  image  on  the  retina 
is  the  same  as  if  there  were  no  fog.  But  the  indistinctness 
with  which  the  object  is  seen  leads  to  the  wrong  conclusion 
that  it  is  situated  at  some  considerable  distance  from  the 
observer.  Hence  the  judgment  is  formed  that  the  object  is 
large,  because  if  it  were  not  large  it  could  not,  at  the  appar- 
ently greater  distance,  produce  an  image  on  the  retina  of 
the  size  it  does. 

The  moon,  or  the  sun,  when  near  the  horizon,  appears 
very  much  larger  than  when  it  is  high  in  the  sky.  This  is 
Usually  said  to  be  due  to  the  fact  that  when  in  the  latter 


47°  ELEMENTARY   PHYSIOLOGY  less. 

position  we  have  nothing  to  compare  it  with,  and  the  small 
extent  of  the  retina  which  its  image  occupies  suggests  small 
absolute  size.  But  as  it  sets,  we  see  it  passing  behind  great 
trees  and  buildings  which  we  know  to  be  very  large  and 
very  distant,  and  yet  it  occupies  a  larger  space  on  the  retina 
than  they  do.  Hence  the  vague  suggestion  of  its  larger 
size.  But  this  has  really  comparatively  little  to  do  with  the 
delusion,  for  the  appearance  is  the  same  if  the  sun  or  moon 
is  seen  near  the  horizon  over  the  open  sea,  where  no  com- 
parison with  other  objects  is  possible.  Probably  one  cause 
of  the  delusion  is  that  when  low  down  the  sun  or  moon  is 
seen  less  distinctly,  on  account  of  mist  and  vapour,  and 
thus  "  looks  "  large  for  the  same  reason  that  a  man  seen  in 
a  fog  appears  unduly  big.  Another  cause  may  be  the  fact 
that  to  most  people  the  distance  from  them  to  the  horizon 
appears  greater  than  the  distance  straight  above  them  to 
the  summit  of  the  vault  of  the  heavens  (or  the  zenith). 
Hence,  though  the  actual  size  of  the  image  of  the  sun  or 
moon  on  the  retina  is  the  same  whether  the  object  be  low 
down  or  high  up,  the  idea  that  it  is  further  off  when  low 
down  suggests  that  it  is  of  greater  size. 

8.  The  Judgment  of  Form  by  Shadows.  —  If  a  convex 
surface  be  lighted  from  one  side,  the  side  towards  the  light 
is  bright  —  that  turned  from  the  light,  dark,  or  in  shadow ; 
while  a  concavity  is  shaded  on  the  side  towards  the  light, 
bright  on  the  opposite  side. 

If  a  new  half-crown,  or  a  medal  with  a  well-raised  head 
upon  its  face,  be  lighted  sideways  by  a  candle,  we  at  once 
know  the  head  to  be  raised  (or  a  cameo')  by  the  disposition 
of  the  light  and  shade ;  and  if  an  intaglio,  or  medal  on 
which  the  head  is  hollowed  out,  be  lighted  in  the  same  way, 
its  nature  is  as  readily  judged  by  the  eye. 

But  now,  if  either  of  the  objects  thus  lighted  be  viewed 


xi  JUDGMENT   OF  CHANGES   OF  FORM  471 

with  a  convex  lens,  which  inverts  its  position,  the  light  and 
dark  sides  will  be  reversed.  With  the  reversal  the  judgment 
of  the  mind  will  change,  so  that  the  cameo  will  be  regarded 
as  an  intaglio,  and  the  intaglio  as  a  cameo ;  for  the  light 
still  comes  from  where  it  did,  but  the  cameo  appears  to 
have  the  shadows  of  an  intaglio,  and  vice  tiersa.  So  com- 
pletely, however,  is  this  interpretation  of  the  facts  a  matter 
of  judgment,  that  if  a  pin  be  stuck  beside  the  medal  so  as 
to  throw  a  shadow,  the  pin  and  its  shadow,  being  reversed 
by  the  lens,  will  suggest  that  the  direction  of  the  light  is  also 
reversed,  and  the  medals  will  seem  to  be  what  they  really 
are. 

9.  The  Judgment  of  Changes  of  Form.  —  Whenever  an 
external  object  is  watched  rapidly  changing  its  form,  a  con- 
tinuous series  of  different  pictures  of  the  object  is  impressed 
upofi  the  same  spot  of  the  retina. 

Conversely,  if  a  continuous  series  of  different  pictures  of 
one  object  is  impressed  upon  one  part  of  the  retina,  the  mind 
judges  that  they  are  due  to  a  single  external  object,  under- 
going changes  of  form. 

This  is  the  principle  of  the  curious  toy  called  the  thau- 
matrope,  or  "  zootrope,"  or  "  wheel  of  life,"  by  the  help  of 
which,  on  looking  through  a  hole,  one  sees  images  of  jug- 
glers throwing  up  and  catching  balls,  or  boys  playing  at 
leap-frog  over  one  another's  backs.  This  is  managed  by 
painting  at  intervals,  on  a  disk  of  card,  figures  and  jugglers 
in  the  attitudes  of  throwing,  waiting  to  catch,  and  catching ; 
or  boys  "  giving  a  back,"  leaping,  and  coming  into  position 
after  leaping.  The  disk  is  then  made  to  rotate  before  an 
opening,  so  that  each  image  shall  be  presented  for  an  in- 
stant, and  follow  its  predecessor  before  the  impression  of 
the  latter  has  died  away.  The  result  is  that  the  succession 
of  different  pictures  irresistibly  suggests  one  or  more  objects 


472  ELEMENTARY   PHYSIOLOGY  less, 

undergoing  successive  changes  —  the  juggler  seems  to  throw 
the  balls,  and  the  boys  appear  to  jump  over  one  another's 
backs.  The  same  explanation  holds  good  for  the  "  animated 
photographs  of  the  kinetoscope  "  (see  p.  451). 

10.  Single  Vision  with  Two  Eyes.  Corresponding 
Points.  —  When  an  external  object  is  ascertained  by  touch 
to  be  single,  the  centres  of  its  retinal  images  in  the  two  eyes 
fall  upon  the  centres  of  the  yellow  spots  of  the  two  eyes,  when 
both  eyes  are  directed  towards  it;  but  if  there  be  two  external 
objects,  the  centres  of  both  their  images  cannot  fall,  at  the 
same  time,  upon  the  centres  of  the  yellow  spots. 

Conversely,  when  the  centres  of  two  images,  formed  simul- 
taneously in  the  two  eyes,  fall  upon  the  centres  of  the  yellow 
spots,  the  mind  judges  the  images  to  be  caused  by  a  single 
external  object ;  but  when  not,  by  two. 

This  seems  to  be  the  only  admissible  explanation  of  the 
facts  that  an  object  which  appears  single  to  the  touch  and 
when  viewed  with  one  eye  also  appears  single  when  it  is 
viewed  with  both  eyes,  though  two  images  of  it  are  neces- 
sarily formed ;  and,  on  the  other  hand,  that  when  the 
centres  of  the  two  images  of  one  object  do  not  fall  on  the 
centres  of  the  yellow  spots  both  images  are  seen  separately, 
and  we  have  double  vision.  In  squinting,  the  axes  of  the 
two  eyes  do  not  converge  equally  towards  the  object  viewed. 
In  consequence  of  this,  when  the  centre  of  the  image  formed 
by  one  eye  falls  on  the  centre  of  the  yellow  spot,  the  corre- 
sponding part  of  that  formed  by  the  other  eye  does  not,  and 
double  vision  is  the  result. 

For  simplicity's  sake  we  have  supposed  the  images  to  fall 
on  the  centre  of  the  yellow  spot.  But  though  vision  is  dis- 
tinct only  in  the  yellow  spot,  it  is  not  absolutely  limited  to 
it ;  and  it  is  quite  possible  for  an  object  to  be  seen  as  a 
single  object  with  two  eyes,  though  its  images  fall  on  the 


xi  JUDGMENT   OF   SOLIDITY  473 

two  retinas  outside  the  yellow  spots.  All  that  is  necessary 
is  that  the  two  spots  of  the  retinas  on  which  the  images  fall 
should  be  similarly  disposed  towards  the  centres  of  their 
respective  yellow  spots.  Any  two  points  of  the  two  retinas 
thus  similarly  disposed  towards  their  respective  yellow  spots 
(or,  more  exactly,  to  the  points  in  which  the  visual  axes  end), 
are  spoken  of  as  corresponding  points  ;  and  any  two  images 
covering  two  corresponding  areas  are  conceived  of  as  com- 
ing from  a  single  object.  It  is  obvious  that  the  inner  (or 
nasal)  side  of  one  retina  corresponds  to  the  outer  (or  cheek) 
side  of  the  other. 

11.  The  Judgment  of  Solidity.  —  When  a  body  of  moder- 
ate size,  ascertained  by  touch  to  be  solid,  is  viewed  with  both 
eyes,  the  images  of  it  formed  by  the  two  eyes  are  necessarily 
different  {one  showing  more  of  its  right  side,  the  other  more 
of  its  left  side) .  Nevertheless,  they  coalesce  into  a  common 
image,  which  gives  the  impression  of  solidity. 

Conversely,  if  the  two  images  of  the  right  and  left  aspects 
of  a  solid  body  be  made  to  fall  upon  the  retinas  of  the  two 
eyes  in  such  a  way  as  to  coalesce  into  a  common  image,  they 
are  judged  by  the  mind  to  proceed  from  the  single  solid  body 
which  alone,  tinder  ordinary  circumstances,  is  competent  to 
produce  them. 

The  stereoscope  is  constructed  upon  this  principle.  What- 
ever its  form,  it  is  so  contrived  as  to  throw  the  images 
of  two  pictures  of  a  solid  body,  such  as  would  be  obtained 
by  the  right  and  left  eye  of  a  spectator,  on  such  parts  of  the 
retinas  of  the  person  who  uses  the  stereoscope  as  would 
receive  these  images  if  they  really  proceeded  from  one 
solid  body.  The  mind  immediately  judges  them  to  arise 
from  a  single  external  solid  body,  and  sees  such  a  solid 
body  in  place  of  the  two  pictures. 

The  operation  of  the  mind  upon  the  sensations  presented 


474  ELEMENTARY   PHYSIOLOGY  less,  xi 

to  it  by  the  two  eyes  is  exactly  comparable  to  that  which 
takes  place  when,  on  holding  a  marble  between  the  finger 
and  thumb,  we  at  once  declare  it  to  be  a  single  sphere 
(p.  461).  That  which  is  absolutely  presented  to  the  mind 
by  the  sense  of  touch  in  this  case  is  by  no  means  the  sensa- 
tion of  one  spheroidal  body,  but  two  distinct  sensations  of 
two  convex  surfaces.  That  these  two  distinct  convexities 
belong  to  one  sphere  is  an  act  of  judgment,  or  process  of 
unconscious  reasoning,  based  upon  many  particulars  of  past 
and  present  experience,  of  which  we  have,  at  the  moment, 
no  distinct  consciousness. 


LESSON  XII 

THE  NERVOUS  SYSTEM  AND  INNERVATION 

1.   The  General  Arrangement  of  the  Nervous  System.  — 

The  sensory  organs  are,  as  we  have  seen,  the  channels 
through  which  particular  physical  agents  are  enabled  to 
excite  the  sensory  nerves  with  which  these  organs  are  con- 
nected ;  and  the  activity  of  these  nerves  is  evidenced  by 
that  of  the  central  organ  of  the  nervous  system,  which 
activity  becomes  manifest  as  a  state  of  consciousness  —  the 
sensation. 

We  have  also  seen  that  the  muscles  are  instruments  by 
which  a  motor  nerve,  excited  by  the  central  organ  with 
which  it  is  connected,  is  able  to  produce  motion. 

The  sensory  nerves,  the  motor  and  other  efferent  nerves, 
and  the  central  organ,  constitute  the  greater  part  of  the 
nervous  system,  which,  with  its  function  of  innervation,  we 
must  now  study  somewhat  more  closely,  and  as  a  whole. 

The  nervous  apparatus  consists  of  two  sets  of  nerves 
and  nerve-centres,  which  are  intimately  connected  together 
and  yet  may  be  conveniently  studied  apart.  These  are  the 
cerebrospinal  system  and  the  sympathetic  system.  The 
former,  or  central  nervous  system,  consists  of  the  brain 
(see  Fig.  i)  and  spinal  cord,  and  the  cranial  and  spinal 
nerves,  which  are  connected  with  the  brain  and  cord.  The 
sympathetic  system  comprises  the  chain  of  sympathetic 
ganglia,  the  nerves  which  they  give  off,  and  the  various  cords 

475 


476 


ELEMENTARY    PHYSIOLOGY 


by  which  they  are  connected  with  one  another  and  with  the 

cerebro-spinal  nerves  (see 


Fig.  147). 

Each  system  consists  of 
nerve-centres  with  their 
connections  among  them- 
selves, and  nerves,  the  lat- 
ter connecting  the  centres 
with  the  other  parts  of  the 
body.  Nerves  are  made 
up  entirely  of  nerve-fibres. 
Nerve-centres,  on  the  other 
hand,  are  composed  of 
nerve-cells  mingled  with 
nerve-fibres  (see  p.  491). 
Nerve-centres  with  their 
connections  form  the  mass 
of  the  brain  and  spinal 
cord ;  they  constitute  also 
the  sympathetic  ganglia, 
the  ganglia  belonging  to 
spinal  nerves,  and  the  gan- 
glia found  in  certain  sensory 
organs,  such  as  the  retina 
and  the  internal  ear. 

2.  The  Investing  Mem- 
branes of  the  Cerebro- 
spinal System. — The  brain 
and  spinal  cord  lie  in  the 
cavity  of  the  skull  and 
spinal  column,  the  bony 
walls  of  which  cavity  are 
lined  by  a  very  tough   fibrous    membrane,  serving   as   the 


Xii  ANATOMY   OF   THE   SPINAL   CORD  477 

periosteum  of  the  component  bones  of  this  region,  and 
called  the  dura  mater.  The  brain  and  spinal  cord  them- 
selves are  closely  invested  by  a  very  vascular  membrane  of 
fibrous  connective  tissue  called  pia  mater.  The  numerous 
blood-vessels  supplying  these  organs  run  for  some  distance 
in  the  pia  mater,  and  where  they  pass  into  the  substance  of 
the  brain  or  cord  the  fibrous  tissue  of  the  pia  mater  accom- 
panies them  to  a  greater  or  less  depth. 

Between  the  pia  mater  and  the  dura  mater  lies  another 
delicate  membrane  called  the  arachnoid  membrane.  These 
three  membranes  are  connected  with  each  other  at  various 
points,  and  the  arachnoid,  which  is  not  only  very  delicate, 
but  also  less  regular  than  the  other  two,  divides  the  space 
between  the  dura  and  pia  mater  into  two  spaces,  each  con- 
taining fluid  and  being  in  connection  with  the  lymphatics, 
and  each  more  or  less  lined  by  a  delicate  epithelium.  The 
space  between  the  dura  mater  and  the  arachnoid,  often  called 
the  subdural  space,  is  nowhere  very  large,  but  the  space 
between  the  arachnoid  and  the  pia  mater,  often  called  the 
subarachnoid  space,  though  small  and  insignificant  in  the 
region  of  the  brain,  becomes  large  in  the  region  of  the  spinal 
cord,  and  here  contains  a  considerable  quantity  of  fluid 
called  cerebro-spinal  fluid.  This  fluid  has  the  appearance 
of  ordinary  lymph,  but  differs  from  it  in  composition.  It 
may  perhaps  serve  as  a  protective  cushion  surrounding  the 
delicate  nervous  mass. 

3.  The  Anatomy  of  the  Spinal  Cord  and  the  Roots  of 
the  Spinal  Nerves. — The  spinal  cord  (Fig.  147)  is  a 
column  of  greyish-white,  soft  substance,  extending  from  the 
top  of  the  spinal  canal,  where  it  is  continuous  by  means  of 
the  spinal  bulb  with  the  rest  of  the  brain,  to  about  the 
second  lumbar  vertebra,  where  it  tapers  off  into  a  filament. 
Starting  at   the  Jevel  of  the  junction  of  the  atlas  vertebra 


478  ELEMENTARY   PHYSIOLOGY  less. 

with  the  skull,  the  spinal  cord  gives  off  laterally  thirty-one 
pairs  of  spinal  nerves,  whose  trunks  pass  out  of  the  spinal 
canal  by  apertures  between  the  vertebrae  called  the  inter- 
vertebral foramina,  and  then  divide  and  subdivide,  their 
ultimate  branches  going  for  the  most  part  to  the  muscles 
and  to  the  skin.  Each  nerve  originates  from  the  cord  by 
two  roots ;  consequently  there  are  twice  as  many  roots  as 
there  are  pairs  of  spinal  nerves  (Fig.  149).  After  their  exit 
from  the  spinal  canal  the  spinal  nerves  become  connected 
with  a  chain  of  ganglia  which  lies  parallel  to  the  spinal 
cord  and  constitutes  the  sympathetic  nervous  system  (Fig. 
147),  which  will  be  described  later  on. 

Transverse  sections  of  the  cord  show  that  a  deep,  some- 
what broad,  fissure,  the  anterior  fissure  (Fig.  148,  1), 
divides  it  in  the  middle  line  in  front,  nearly  down  to  its 
centre  :  and  a  similar  deeper  but  narrower  cleft,  the  pos- 
terior fissure  (Fig.  148,  2),  also  extends  nearly  to  its  centre 
in  the  middle  line  behind.  Each  of  these  fissures  extends 
throughout  the  whole  length  of  the  cord.  The  pia  mater 
extends  more  or  less  into  each  fissure,  and  supports  the 
vessels  which  supply  the  cord  with  blood.  In  consequence 
of  the  presence  of  the  fissures  only  a  narrow  bridge  of  the 
substance  of  the  cord  connects  its  two  halves,  and  this 
bridge  is  traversed  throughout  its  entire  length  by  a  minute 
canal,  the  central  canal  of  the  cord  (Fig.  148,  3). 

The  lines  of  origin  of  the  roots  of  the  spinal  nerves  divide 
the  cord  longitudinally  into  three  parts,  called  respectively 
the  anterior,  lateral,  and  posterior  columns  (Fig.  148,  8,  7, 
6),  those  roots  which  arise  along  the  line  which  is  nearer 
the  anterior  surface  of  the  cord  being  known  as  the  anterior 
roots  ;  those  which  arise  along  the  other  line  are  the  pos- 
terior roots  (Figs.  148  and  149).  Each  root  arises  by 
numerous  filameats,  which  leave  the  cord  on  each  side,  con- 


ANATOMY   OF  THE   SPINAL  CORD 


479 


verge  on  the  same  level,  aud  form  the  roots  proper,  and 
then  the  latter,  anterior  and  posterior,  coalesce  on  each  side 
into  the  trunk  of  a  spinal  nerve  ;   but  before  doing  so  the 


(MA  u 

uli  U~* 


Fig.  148.  —  Transverse  Section   of  One-halk  of  the  Spinal  Cord   (in  the 
/     — f  _  .  .. Ll'.mbak,  Region)   magnified. 

1,  anterior  hssure;  2,  posterior  fissure:  3,  central  canal-,  4,  and  5,  bridges  connect- 
ing the  two  halves  (posterior  and  anterior  commissures):  6,  posterior  column;  7, 
lateral  column;   8,  anterior  column;   9,  posterior  root;  10,  anterior  root  of  nerve. 

a  a,  posterior  horn  of  grey  matter;  e  e  e,  anterior  horn  of  grey  matter.  Through 
the  several  columns  6,  7,  and  8,  each  composed  of  white  matter,  are  seen  the  prolonga- 
tions of  the  pia  mater,  which  carry  blood-vessels  into  the  cord  from  the  outside.  The 
pia  mater  itself  is  seen  surrounding  the  whole  of  the  cord. 

posterior    root    presents    an    elongated     enlargement — the 
ganglion  of  the  posterior  root  (Fig.  149,  Gn.). 

A  transverse  section  of  the  spinal  cord   (Fig.  149,  B,  and 


480 


ELEMENTARY   PHYSIOLOGY 


Fig.  148)  shows,  further,  that  each  half  consists  of  two 
substances  —  a  white  matter  on  the  outside,  and  a  greyish- 
red  substance  in  the  interior.  And  this  grey  matter,  as  it  is 
called,  is  so  disposed  that  in  a  transverse  section  it  looks, 
in  each  half,  something  like  a  crescent,  with  one  end  bigger 
than  the  other,  and  with  the  concave  side  turned  outwards. 
The  two  ends  of  each  crescent  are  called  its  horns  or  cornua, 
the  one  directed  forwards  being  the  anterior  cornu  (Fig. 
148,  e  <?)  ;  the  one  turned  backwards  the  posterior  cornu 
(Fig.  148,  a  a).  The  convex  sides  of  the  crescents  of  the 
grey  matter  approach  one  another,  and  are  joined  by  the 
bridge  which  contains  the  central  canal. 


A  B 

Fig.  149.  —  The  Spinal  Cord. 

A.  A  front  view  of  a  portion  of  the  cord.  On  the  right  side,  the  anterior  roots, 
A.R,  are  entire;  on  the  left  side  they  are  cut,  to  show  the  posterior  roots,  P.R. 

B.  A  transverse  section  of  the  cord.  A,  the  anterior  fissure;  P,  the  posterior  fis- 
sure; G,  the  central  canal;  C,  the  grey  matter;  IF,  the  white  matter;  A.R,  the 
anterior  root,  P.R,  the  posterior  root,  Gn,  the  ganglion,  and  T,  the  trunk,  of  a  spinal 
nerve. 


The  reader  should  bear  clearly  in  mind  the  fact  that  both 
the  grey  and  the  white  matter  extend,  arranged  approxi- 
mately as  just  described,  throughout  the  whole  length  of  the 
spinal  cord,  and  hence  that  each  forms  a  great  columnar 
mass,  the  grey  column  lying  within  the  white  column  and 
having  the  double  crescent  or  H-shape  in  cross-section  only. 

There  is  a  fundamental  difference  in  structure  between 
the  grey  and  the  white  matter.  The  white  matter  consists 
of  nerve-fibres  supported  in  a  delicate  framework  of  con- 


Xii      STRUCTURAL   ELEMENTS   OE  NERVOUS   TISSUE    481 

nective  tissue,  and  accompanied  by  blood-vessels.  The 
grey  matter,  on  the  other  hand,  contains  in  addition  multi- 
tudes of  nerve-cells,  some  of  them  of  considerable  size. 

Most  of  the  nerve-fibres  of  which  the  anterior  roots  are 
composed  may  be  traced  into  the  anterior  cornu,  and, 
indeed,  into  the  nerve-cells  lying  in  the  cornu,  while  those 
of  the  posterior  roots  enter  or  pass  towards  the  posterior 
cornu. 

4.  The  Structural  Elements  of  Nervous  Tissue.  —  Before 
proceeding  further  in  our  study  of  the  gross  anatomy  of  the 
nervous  system  we  may  advantageously  consider  the  struc- 
tural elements  of  nervous  tissue  and  their  relationships.  So 
far  we  have  spoken  of  nerve-cells  and  nerve-fibres  as  distinct 
things.  As  a  matter  of  fact  they  are  not  distinct,  a  fibre 
being  really  a  part  of  a  cell. 

The  structural  element  of  nervous  tissue  is  the  nerve-cell, 
or,  as  it  is  now  becoming  customary  to  term  it,  the  neuron 
(Fig.  150,  A).  The  neuron  consists  of  a  protoplasmic  cell- 
body  containing  usually  a  prominent  nucleus,  and  giving 
off  from  its  surface  one  or  more  processes.  The  cell-body 
varies  greatly  in  shape  and  in  size  (between  4  and  140/x). 
The  processes  originate  as  outgrowths  from  the  body  of  the 
cell,  and  vary  in  number :  when  one  only  is  present,  the 
cell  is  called  unipolar ;  when  two,  bipolar ;  when  many, 
multipolar.  The  processes  may  be  of  two  kinds,  called 
protoplasmic  processes  or  dendxites  and  axis-cylinder  pro- 
cesses, which  are  also  called  neurites  or  ueuraxes  or  axons. 
Dendrites  are  mere  undifferentiated  extensions  of  the  pro- 
toplasmic body ;  they  are  comparatively  short,  they  branch 
much  and  irregularly,  and  when  present  at  all  are  usually 
numerous  for  each  cell.  Axis-cylinder  processes  are  usually 
much  longer,  reaching  in  some  cases  in  man  probably  a 
length  of  three  feet ;  they  pursue  a  fairly  straight  course  ami 


482 


ELEMENTARY    PHYSIOLOGY 


are  comparatively  little  branched  ; 


Cell-body 


Dendrites 


Medulla. 


Node 


Fig.  150.  —  Diagrams  of  Nerve- 
cells 

A,  a  typical  neuron  ;  in  addition 
to  the  parts  shown  the  neurilemma 
begins  below  the  beginning  of  the 
medulla  and  continues  as  the  ex- 
ternal sheath  of  the  nerve-fibre  to 
the  peripheral  termination  of  the 
latter. 

B,  the  nervous  elements  medi- 
ating a  simple  reflex  action;  s, 
sensory  surface;    ;//,  muscle-fibre. 

The  arrows  indicate  the  direc- 
tion of  conduction, 


they  are  usually  one  in 
number  for  each  cell : 
and  they  are  highly  dif- 
ferentiated in  structure, 
being  apparently  com- 
posed of  minute  fibrillge. 
At  a  greater  or  less  dis- 
tance from  the  cell-body 
each  axis-cylinder  pro- 
cess becomes  the  axis- 
cylinder,  or  essential 
part,  of  a  nerve-fibre. 
No  nerve  -  fibre  exists 
that  is  not  an  outgrowth 
from,  and  hence  a  part 
of,  a  nerve-cell. 

If  a  nerve-fibre  be  cut 
off  from  the  cell-body 
to  which  it  belongs,  it 
gradually  dies  and  de- 
generates, and  its  axis- 
cylinder  ultimately  dis- 
appears. This  is  doubt- 
less due  to  its  removal 
from  the  influence  of 
the  nucleus  of  the  cell, 
since  it  has  been  shown 
by  numerous  experi- 
ments on  unicellular  or- 
ganisms that  the  nucleus 
is  necessary  to  the  con- 
tinued life  of  all  parts 
of  the  cell.   Through  the 


xii  TILE   STRUCTURE   OF   NERVES  483 

influence  of  the  nucleus  the  cell-body  is  supposed  to  exert 
constantly  some  obscure  kind  of  nutritional  action  over  the 
axis-cylinder  process. 

The  general  function  of  the  structural  elements  of  nervous 
tissue  is  that  of  generating  nervous  impulses  and  conduct- 
ing them  from  one  part  of  the  nervous  system  to  another. 
By  the  aid  of  the  proper  stimuli  such  impulses  are  gen- 
erated either  in  the  cell-body  or  at  the  ends  of  the  pro- 
cesses. Although  it  is  probable  that  the  dendrites  conduct 
nervous  impulses  to  the  cell-body,  the  axis-cylinder  process 
is  the  pre-eminently  conducting  part  of  the  neuron.  In  dif- 
ferent neurons  it  may  conduct  either  towards  or  away  from 
the  cell-body,  but  in  any  one  neuron  throughout  life  it  con- 
ducts always  in  the  same  direction ;  when  a  single  axon  is 
present,  it  conducts  always  away  from  the  cell-body. 

In  their  passage  from  one  part  of  the  nervous  system  to 
another  nervous  impulses  pass  from  one  neuron  to  another. 
The  recent  abundant  work  on  the  structure  of  the  nervous 
system  seems  to  show  that  there  is  no  actual  direct  connec- 
tion of  one  neuron  with  another,  but  that  the  neurite  of  one 
terminates  in  a  number  of  filaments  which  surround  and 
interlace  with  the  dendrites  or  cell-body  of  another  neuron 
(Figs.  150,  B,  and  178),  just  as  we  have  seen  the  various 
cells  within  the  retina  to  communicate  with  one  another  by 
terminal  filaments  that  touch  but  do  not  join. 

5.  The  Structure  of  Nerves.  —  If  a  small  piece  of  a 
nerve,  which  may  be  easily  obtained  from  the  leg  of  a 
freshly  killed  frog  or  rabbit,  be  teased  out  with  needles  on 
a  glass  slide  and  examined  under  a  microscope  it  is  seen  to 
be  made  up  chiefly  of  minute  fibres  When  the  nerve  has 
been  suitably  hardened,  it  becomes  possible  to  cut  a  trans- 
verse section  of  it  ;  if  this  section  be  similarlv  examined, 
the  cut  ends  of  the  fibres  may  be  readily  seen  as  little  circu- 


4^4 


ELEMENTARY    PHYSIOLOGY 


lar  dots  arranged  in  groups  which  compose  the  larger  part 
of  the  section  (Fig.  151,  /).  The  fibres  are  bound  together 
in  bundles,  which  are  rounded,  as  seen  in  section,  by  an 
external  sheath  or  case  of  connective  tissue  called  the  peri- 
neurium, per  (analogous  to  the  perimysium  which  binds 
muscle-fibres  together),  from  whose  inner  surface  very  deli- 
cate layers  of  connective  tissue  pass  in  between  the  fibres 
of  which  each  bundle  is  composed.  The  several  bundles 
are  themselves  bound  together  by  connective  tissue  to  form 
the  trunk  of  the  nerve,  and  the  whole  nerve,  thus  built  up 


Fig.  151. —Transverse  Section  of  a  Medium-sized  Medullated  Nerve. 

ep,  ep,  general  connective-tissue  sheath  or  epineurium;  /,/,/,  bundles  of  nerve- 
fibres  bound  together  by  the  perineurium, per, per,  per;  A, A,  V,  blood-vessels;  L, 
lymphatic  vessel. 

of  bundles  of  nerve-fibres,  is  surrounded  and  held  together 
by  an  external  layer  of  connective  tissue. 

The  nerve-fibres,  which  are  the  essential  elements  of  the 
nerve,  vary  in  diameter  from  2//,  to  16/x,  or  more.  In  the 
living  state  they  are  very  soft  cylindrical  rods  of  a  glassy, 
rather  strongly  refracting  aspect.  Running  through  the 
centre  of  the  rod,  a  band   of  somewhat  less  transparency 


xii  THE   STRUCTURE   OF   NERVES  485 

than  the  rest  may  be  discerned.  At  intervals,  the  length 
of  which  varies,  but  is  always  many  times  greater  than  the 
thickness  of  the  rod,  the  nerve-fibre  presents  sharp  constric- 
tions, which  are  termed  nodes  (Fig.  152,  A,  n  ;  B,  n  n). 
Somewhere  in  the  interspace  between  every  two  nodes,  very 
careful  examination  will  reveal  the  existence  of  a  nucleus 
(Fig.  152,  B,  nc),  invested  by  more  or  less  granular  proto- 
plasmic substance  and  lying  in  the  substance  of  the  rod,  but 
close  to  the  surface. 

As  the  fibre  dies,  and  especially  if  it  is  treated  with 
certain  reagents,  these  appearances  rapidly  change.  1.  The 
outermost  layer  of  the  fibre  becomes  recognisable  as  a  defi- 
nite membrane,  the  neurilemma  ]  (the  so-called  "  primitive 
sheath"  or  "sheath  of  Schwann").  2.  The  central  band 
becomes  more  opaque,  and  sometimes  appears  marked  with 
fine  longitudinal  striae  as  if  it  were  composed  of  extremely 
fine  fibrillar;  it  is  the  axis-cylinder  or  neuraxis  (Fig.  152, 
nx).  3.  Where  the  axis-cylinder  traverses  one  of  the  nodes, 
the  neurilemma  is  seen  to  embrace  it  closely,  but  in  the 
intervals  between  the  nodes  a  curdy-looking  matter,  which 
looks  white  by  reflected  light,  occupies  the  space  between 
the  neurilemma  and  the  axis-cylinder.  This  is  the  medulla 
(the  so-called  "white  substance  of  Schwann")  largely 
composed  of  a  complex  fatty  substance  often  spoken  of  as 
myelin.  If  the  neurilemma  of  a  fresh  fibre  is  torn,  the 
myelin  flows  out  and  forms  irregular  lumps  as  if  it  were  vis- 
cous.    4.  The  internodal  nucleus  is  more  sharply  defined  ; 

1  This  word  was  formerly  used  to  denote  the  sheath  of  a  bundle  of 
nerve-fibres,  now  called  perineurium ;  but  its  similarity  to  the  word  sarco- 
lemma  led  to  great  confusion  in  the  minds  of  students.  It  is  undoubtedly 
a  wholesome  rule  never  to  use  an  old  word  in  a  new  sense;  but  the  striking 
similarity  between  the  two  words  "  neurilemma  "  and  "  sarcolemma,"  and 
between  the  nerve-fibre  sheath  and  the  muscle-fibre  sheath,  seems  an  ade- 
quate excuse  for  an  exception  to  the  rule. 


486 


ELEMENTARY   PHYSIOLOGY 


and  it  will  be  seen  to  be  attached  to  the  inner  surface  of 
the  neurilemma. 

The  essential  part  of  each  fibre,  regarded  as  an  instrument 
for  the  transmission  of  that  molecular  disturbance  which  is 


Fig.  152.  —To  illustrate  the  Structure  of  Nerve-fibres. 

A.  A  nerve-fibre  seen  without  the  use  of  reagents,  showing  the  "  double  contour  " 
due  to  the  medulla,  and  u,  a  node.  Neither  neuraxis  nor  neurilemma  can  be  distinctly 
seen.     (Magnified  about  300  diameters.) 

B.  A  thin  nerve-fibre  treated  with  osmic  acid,  showing  nc,  nucleus  with  granular 
protoplasm,/,  surrounding  it,  beneath  the  neurilemma;  n,  11,  the  two  nodes  marking 
out  the  segment  to  which  the  nucleus  belongs.     (Magnified  400  diameters.) 

C.  Portion  of  fibre  (thicker  than  B)  treated  with  osmic  acid  to  show  the  node  n; 
m,  the  densely  stained  medulla;  at  in'  the  medulla  is  seen  divided  into  segments. 
(Magnified  350  diameters. ) 

D.  Portion  of  nerve-fibre  treated  to  show  the  passage  of  the  neuraxis,  nx,  through 
the  node,  n :  ///,  the  medulla.  At  nx'  the  neuraxis  is  swollen  by  the  reagents  em- 
ployed, and  is  large  and  irregular.      (Magnified  300  diameters.'* 

E.  Portion  of  nerve-fibre  treated  with  osmic  acid,  showing  the  nucleus,  nc,  im- 
bedded in  the  medulla:  c,  fine  perineuria!  sheath  lying  outside  the  neurilemma;  the 
outline  of  the  latter  can  only  be  recognised  over  the  nucleus,  nc;  the  nucleus,  nc' , 
belongs  to  this  perineurial  sheath.     (Magnified  400  diameters.) 

F.  Portion  of  nerve-fibre  deprived  of  its  neurilemma  and  showing  the  medulla 
broken  up  into  separate  fragments,  in,  in,  surrounding  the  neuraxis,  nx. 


xii  THE   STRUCTURE   OF  NERVES  487 

spoken  of  as  a  "  nervous  impulse,"  is  the  axis-cylinder. 
This  is  proved  by  the  fact  that  the  axis-cylinder  alone  pro- 
vides the  actual  connection  between  the  central  nervous 
system  and  the  distant  structures  to  or  from  which  the 
motor  (efferent)  or  sensory  (afferent)  nerves  run.  Thus, 
if  we  follow  along  the  course  of  a  motor  nerve,  proceeding 
to  its  muscle,  we  find  that  it  enters  the  perimysium  (with 
which  the  superficial  layer  of  the  perineurium  becomes  con- 
tinuous), and  divides  in  the  perimysial  septa  into  smaller 
and  smaller  branches,  each  of  which  contains  the  continua- 
tion of  a  certain  number  of  the  fibres  of  the  nerve  trunk, 
bound  up  into  a  bundle  by  themselves.  In  these  larger 
ramifications  of  the  nerve  trunk  there  is  no  branching  of  the 
nerve-fibres  themselves  (at  any  rate  as  a  rule),  but  merely  a 
separation  of  the  fibres  of  the  compound  nerve  bundles. 
In  the  finer  branches,  however,  the  nerve-fibres  themselves 
may  divide  ;  the  division,  which  always  takes  place  at  a 
node,  is  generally  dichotomous  —  that  is,  one  fibre  divides 
into  two,  each  of  these  again  into  two,  and  so  on.  An  ulti- 
mate branch  consisting  of  one  or  two  nerve-fibres,  or  of  one 
only,  with  a  very  delicate  connective-tissue  envelope  (Fig. 
152,  E,  c),  passes  to  some  single  muscle-fibre,  and  each 
nerve-fibre  applies  itself  to  the  outer  surface  of  the  sarco- 
lemma.  At  this  point,  if  it  has  not  done  so  before,  the 
medulla  disappears,  the  neurilemma  becomes  continuous  with 
the  sarcolemma,  and  the  axis-cylinder  breaks  up  into  short 
irregular  branches  which,  ending  abruptly,  are  applied  to  a 
disc  of  protoplasmic  substance  containing  many  nuclei,  thus 
forming  what  is  called  a  motor  end-organ  or  end-plate,1 
which  is  interposed  between  the  striated  muscle  substance 
and  the  sarcolemma  at  this  point.     The  exact  relations  of 

1  This  is  the  arrangement  in  most  vertebrated  animals.     In  the  frog  the 
axis-cylinder  branches  out  without  entering  a  distinct  motor  end-plate. 


4SS  ELEMENTARY    PHYSIOLOGY  less. 

the  various  parts  of  the  end-plate  to  the  muscle  substance 
have  not  yet  been  clearly  made  out.  The  whole  appear, 
however,  to  constitute  an  apparatus  by  which  the  molecular 
disturbances  of  the  substance  of  the  axis-cylinder  (the  essen- 
tial part  of  the  nerve)  may  be  efficiently  propagated  to  the 
substance  of  the  muscle. 

If,  instead  of  following  the  motor  nerve  to  its  distribution 
in  the  muscle,  we  trace  it  the  other  way,  towards  the  spinal 
cord,  we  shall  find  no  alteration  of  any  moment  until  we 
arrive  at  the  point  at  which  the  anterior  root  enters  the 
cord.  From  the  finest  branches  of  the  motor  nerve  (in 
which,  as  has  been  stated,  the  nerve-fibres  themselves 
divide)  to  this  point  of  entry  each  nerve:fibre  extends  en- 
sheathed  as  one  continuous  undivided  axis-cylinder  in  a  long 
succession  of  internodal  segments.  At  the  point  of  entry  into 
the  cord  the  perineurium  passes  into  the  pia  mater  and  the 
general  connective-tissue  framework  of  the  cord.  The  neu- 
rilemma and  the  nodes  disappear.  Often  the  axis-cylinder 
can  be  traced  towards  the  anterior  horn  of  the  grey  matter, 
invested  only  by  a  sheath  of  medulla,  which  gradually  be- 
comes thinner  and  thinner  until  at  length  it  disappears,  and 
the  fibre,  thus  reduced  to  its  axis-cylinder,  passes  into  one 
of  the  large  nerve-cells  which  lie  in  the  anterior  cornu  of 
the  grey  matter  (see  p.  491). 

The  axis-cylinder  of  a  motor  nerve-fibre,  therefore,  is  an 
extremely  fine  and  long  process  of  a  nerve-cell,  which  passes 
at  its  peripheral  end  into  one  or  more  muscle-fibres :  in  other 
words,  the  body  of  the  nerve-cell  and  the  muscle-cells  are 
the  central  and  peripheral  end-organs  of  the  nerve-fibre. 

With  one  or  two  exceptions,  sensory  (afferent)  nerve- 
fibres  are  not  distinguishable  by  any  structural  character 
from  motor  nerve-fibres.  Wherever  special-sense  organules 
(]>.  372)  exist,  the  sensory  fibres  are  connected  with  them 


THE   STRUCTURE  OF  NERVES 


489 


by  means  of  their  axis-cylinder,  from  which  the  neurilemma 
and  medulla  have  disappeared.  If,  as  before,  we  follow  the 
sensory  nerve-fibres  back  towards  the  spinal  cord,  we  find 
that  they  pass  through  the  ganglion  on  one  of  the  posterior 
roots,  and  then  enter  the  substance  of  the  cord,  passing 
towards  the  posterior  cornu.  Like  the  motor  nerve-fibres, 
they  lose  their  noded  neurilemma  as  they  enter  the  cord,  so 
that  in  this  case  also  it  is  again  the  axis-cylinder  which  pro- 
vides the  actually  continuous  connection  between  the  sense- 
organ  and  the  central  nervous  system. 

The  neurilemma,  with  its  nucleus  and  the  medulla,  may 
be  regarded  as  a  covering  which  provides  for  the  protection 
and  nourishment  of  each  successive  length  of  the  essentially 
important  neuraxis  or  axis-cylinder. 


Fig.   153.  —  Pale  non-Meduxlated  Fibres  from   the  Pneumogastric  Nerve 
(Ranvier). 

«,  nucleus;  /,  protoplasm  belonging  to  the  nucleus. 

The  fibres  which  make  up  the  essential  structure  of  the 
nerves  with  which  we  have  so  far  dealt  are  spoken  of  as 
medullated,  because  except  at  their  peripheral  and  central 
terminations  they  possess  the  characteristic  medulla  (p.  485). 
But  scattered  among  these  medullated  fibres  are  a  few  which 
are  often  spoken  of  as  non-medullated,  because  they  possess 
no  medulla.  These  non-medullated  fibres  are  peculiarly 
abundant  in  the  nerves  of  the  sympathetic  system,  so  much 
so   that    they  are    frequently  called    "  sympathetic  fibres." 


49Q  ELEMENTARY   PHYSIOLOGY  "less. 

They  appear  under  the  microscope  as  pale  flattened  bands, 
about  as  wide  as  small  medullated  fibres,  often  fibrillated 
longitudinally,  and  frequently  dividing  (Fig.  153).  They 
appear,  in  fact,  to  be  naked  axis-cylinders,  without  medulla, 
and  apparently  without  a  neurilemma,  though  they  bear  at 
intervals  on  their  surface  nuclei,  which  may  represent  the 
internodal  nuclei  of  ordinary  nerve-fibres. 

6.  The  Minute  Structure  of  the  Spinal  Cord  and  Spinal 
Ganglia.  —  The  white  matter  of  the  spinal  cord  consists 
mainly  of  medullated  nerve-fibres  running  for  the  most 
part  lengthwise.  In  a  transverse  section  the  fibres  hence 
show  their  cut  ends,  and  the  white  matter  appears  to  con- 
sist of  multitudes  of  minute  rings  (medulla),  each  contain- 
ing a  dot  (axis-cylinder).  The  nerve-fibres  are  supported 
by  a  fine  felt-work  of  extremely  delicate  fibres  which  con- 
stitutes what  is  known  as  the  neuroglia  (vevpov  =  nerve,  and 
yXta  =  glue),  since  it  binds  the  nerve-fibres  together.  The 
fibres  of  the  neuroglia  are,  in  reality,  processes  from  number- 
less minute  cells,  the  neuroglia-cells  (Fig.  154),  in  each  of 
which  the  body  of  the  cell  is  extremely  small,  and  the  pro- 
cesses unusually  numerous. 

At  frequent  intervals  all  over  the  surface  of  the  cord  and 
in  the  fissures,  the  pia  mater  sends  conspicuous  longitudinal 
partitions  (Fig.  148),  composed  of  connective  tissue,  into 
the  substance  of  the  white  matter.  These  partitions  divide 
and  subdivide  as  they  extend  farther  into  the  cord  ;  they 
carry  blood-vessels  and  lymphatics  and  provide  for  the  inti- 
mate distribution  of  these  vessels  throughout  the  nervous 
tissue.  The  connective  tissue  and  neuroglia  are  continued 
on  into  the  grey  matter. 

The  most  striking  feature  of  the  grey  matter  is  the  pres- 
ence in  its  neuroglia  of  nerve-cells,  many  of  which  are  very 
large  and  conspicuous,  while  others  are  smaller,  but   still 


xii        MINUTE   STRUCTURE   OF  THE   SPINAL   CORD       491 

very  evident ;  these  cells  and  their  processes,  together  with 
the  comparative  absence  of  medullated  nerve-fibres,  and  the 
presence  of  a  closely  interwoven  network  of  non-medullated 
nerve-fibres,  form  the  chief  contrast  between  the  structure 
of  the  grey  and  the  white  matter  of  the  spinal  cord. 

The  Cells  of  the  Grey  Matter. — These  cells  are  not  scat- 
tered uniformly  throughout  the  grey  matter,  but  are  arranged 
in  groups.  The  largest  cells  occur  at  the  end  of  the  anterior 
horn  (see  Figs.  148  and  156),  and  since  these  are  typical, 
as  regards  the  main  features  of  their  structure,  of  all  the 
cells  of  the  grey  matter,  we  may  take  one  of  them  for 
detailed  description. 


Fig.  154.  —  A  Neuroglia-cell  from  the  White  Matter  of  the  Spinal  Core 
(Schafer). 

The  body  and  processes  of  the  cell  appear  black,  since  they  were  deeply  stained  in 
order  to  bring  out  their  details. 


The  body  of  each  cell  is  large  (varying  in  diameter  from 
50/A  to  140/x;  T^¥  to  yJr-  of  an  inch),  and  contains  a  very 
conspicuous  nucleus  (Fig.  155).  The  cell-body  is  pro- 
longed into  a  varying  number  of  dendrites  (usually  many), 
dividing  and  subdividing  into  branches,  which  may  be  traced 
to  some  distance  from  the  cell,  becoming  finer  and  finer,  and 
finally  ending.  Besides  these  branching  processes  the  cell 
bears  one  axis-cylinder  process,  which  does  not  divide  in 
this  way,  passes  straight  away  from  the  cell,  and  is  soon  cov- 
ered by  a  layer  of  myelin  or  a  medulla  ;  after  its  exit  from 


492 


ELEMENTARY   PHYSIOLOGY 


the  cord,  it  acquires  additionally  a  neurilemma  or  primitive 
sheath.  In  this  way  this  process  becomes  the  axis-cylinder 
or  neuraxis  of  a  medullated  nerve-fibre,  and  is  continuous 
to  the  organ,  usually  a  muscle,  to  which  it  is  distributed. 


Fig.    155.  —  Diagram   of    a   Typical  Cell  from  the  Grey  Matter  of  the 
Spinal  Cord  (Sherrington). 

n,  nucleus;  d,  d,  d,  branched  processes  (dendrites)  from  the  cell-body;  /,  pig- 
ment; c,  part  of  cell-body  which  stains  very  readily  (chromophilic  substance);  a, 
axis-cylinder  process,  or  neuraxon,  which  acquires  first  a  medulla,  m,  and  then  (out- 
side the  cord)  a  neurilemma. 

A,  represents  the  processes  (dendrites)  from  a  neighbouring  cell  interlacing  with, 
but  not  joined  on  to,  the  processes  of  the  cell  figured. 

The  arrows  indicate  the  direction  in  which  the  processes  conduct  nerve  impulses. 


The  Differences  in  Structure  of  the  Spinal  Cord  at  Various 
Levels. — These  differences  show  themselves  most  conspicu- 
ously with  respect  to  (i)  the  shape  of  the  column  of  grey  mat- 
ter at  various  levels,  (ii)  the  position  of  the  chief  groups  of 
nerve-cells  in  the  grey  matter,  and  (iii)  the  amount  of  white 
matter  relatively  to  the  grey  matter  at  each  level.  The  cord 
is  largest  in  the  cervical  region,  smallest  in  the  thoracic 
(dorsal)  region,  and  increases  in  size  again  in  the  lumbar 
region  ;  that  is,  it  is  large  in  those  parts  which  supply  with 
nerves  not  simply  the  portions  of  the  trunk  of  the  body  that 
lie  at  those  levels,  but  the  arms  and  legs  in  addition.     The 


MINUTE   STRUCTURE   OF  THE   SPINAL   CORD       403 


^'  ^  S  2  S 

t/i   J5    O 

v    u   S  —   t- 

>S  E  no 

t  ~  a  i:'  = 

c  £  u  =  ii 


x-'  rt.«  u 


4)  a 


.3    U  .  £    tf.  — 

■a  «  S .  =  o 

*  =  *  ■  g » 

2  "3  *  «  u 

si    i_    U    3  Tl 

U    O    =   5J3 
~'F    "J  "J 

.  n  2 --3 

S  g-fu  m 

-  cj  -  a  o 


u    I-    u      --3 

|2B.o  :J 

j=  n  c  «  5 


U  0 

S'^il: 

J  0 

<;  w 

>  "^  c  *^  g 

«« 

en  « 

|-«sfii§ 

S  P. 

«s«|= 

H  3 

< 

«~  £  J  JJ 

w 

U   0   3   1   3 

h 

-    ='g-^2 

-  k  o  °  ii 

"aj         ^   CJD  u 


494 


ELEMENTARY    PHYSIOLOGY 


chief  structural  differences  are  very  clearly  indicated  in  Fig- 
ure 156,  which  represents  sections,  drawn  to  scale,  of  (half) 
the  spinal  cord  at  the  level  of  A,  the  sixth,  cervical,  B, 
the  sixth,  thoracic  (dorsal),  and  C,  the  third  lumbar  spinal 
nerves  respectively. 

The  Structure  of  a  Spinal  Ganglion.  —  A  spinal  ganglion  is, 
as  we  have  said  (Fig.  149,  G11.),  an  elongated  swelling  on 
the  posterior  root  of  a  spinal  nerve.  In  a  longitudinal  sec- 
tion it  is  seen  to  consist  of  an  external  sheath  of  connective 
tissue  which  incloses  groups  of  large  nerve-cells,  of  which 
the  largest  group  lies  at  its  outer  side.  The  nerve-fibres 
which  enter  the  distal  end  of  the  ganglion  on  their  way  to 


Fig.   157. — A  Nerve-cell  from  the  Ganglion  on  the  Posterior  Root  of  a 
Spinal  Nerve. 

n.c,  the  body  of  the  nerve-cell,  with  n,  nucleus,  ;;',  nucleolus,  /,  protoplasmic 
body;  c,  capsule  of  the  nerve-cell;  «",  nuclei  of  the  capsule;  n.f,  the  nerve -fibre 
which,  at  the  node,  d,  divides  into  two.  At  a  the  neuraxis  of  the  fibre  is  lost  in  the 
substance  of  the  cell;  at  b  it  acquires  a  medulla;  at  «'"  nuclei  are  seen  on  the  fibre. 
At  the  division  the  neuraxis,  d,  is  seen  to  divide,  and  besides  the  neurilemma,  «./,  the 
fibre  has  an  additional  sheath,  s,  continuous  with  the  capsule  of  the  nerve-cell. 

the  spinal  cord  pass  in  bundles  in  between  the  groups  of 
nerve-cells,  and  a  certain  amount  of  connective  tissue,  with 
accompanying  blood-vessels  and  lymphatics,  also  passes  in 
amongst  the  nerve-cells  and  nerve-fibres.  Each  nerve-cell 
(Fig.  157)  consists,  like  a  nerve-cell  of  the  spinal  cord,  of 
a  large  nucleus,  with  a  nucleolus,  and  of  a  cell-body ;  but 
the  cell-body  is,  in  most  cases  at  all  events,  prolonged  into 


xii  FUNCTIONS  OF  ROOTS  OF  SPINAL  NERVES         495 

one  process  only,  so  that  the  whole  cell  is  pear-shaped. 
This  process  soon  acquires  a  medulla  and  a  neurilemma ;  it 
thus  becomes  an  ordinary  medullated  nerve-fibre,  which 
then  divides  into  two  fibres,  one  of  which  may  be  traced 
into  the  nerve  trunk,  and  the  other  along  the  posterior  root 
to  the  spinal  cord.  Hence  the  nerve-cells  of  the  ganglion 
appear  to  be  lateral  appendages  of  the  nerve-fibres,  forming 
a  junction  with  them  after  the  fashion  of  a  T-piece.  On  the 
central,  or  cord,  side  of  the  ganglion  the  fibres  continue 
their  course  into  the  substance  of  the  spinal  cord  towards 
the  posterior  horn.  Like  the  motor  nerves  they  lose  their 
neurilemma  as  they  join  the  cord.  Some  of  them  pass  on 
at  once  into  the  grey  matter  of  the  posterior  horn ;  but  the 
majority  turn  aside  as  they  enter  the  cord  and  run  upwards 
for  some  distance  in  the  posterior  column  of  the  white  sub- 
stance before  they  enter  the  grey  matter. 

Structurally  we  may  regard  the  nerve-fibres  of  the  pos- 
terior roots  of  the  spinal  cord  as  taking  their  origin  from 
one  process  of  a  cell  in  the  spinal  ganglion  in  the  same 
way  that  the  fibres  of  the  anterior  root  originate  in  one 
process  of  a  cell  in  the  anterior  horn  of  the  grey  matter. 
This  accounts  for  the  peculiar  way  in  which  the  fibres  of 
the  posterior  root  make  their  connection  with  the  cord,  and 
also  for  the  most  obvious  function  of  the  spinal  ganglia,  of 
which  we  shall  speak  presently. 

7.  The  Functions  of  the  Roots  of  the  Spinal  Nerves. 
—  The  physiological  properties  of  the  organs  now  described 
are  very  remarkable. 

If  the  trunk  of  a  spinal  nerve  be  irritated  in  any  way 
(at  x  in  Fig.  158),  as  by  pinching,  cutting,  galvanising,  or 
applying  a  hot  body,  two  things  happen  :  in  the  first  place, 
all  the  muscles  to  which  filaments  of  this  nerve  are  dis- 
tributed  contract ;    in   the  second,    pain   is   felt,   and   the 


496  ELEMENTARY   PHYSIOLOGY  less. 

pain  is  referred  to  that  part  of  the  skin  to  which  fibres 
of  the  nerve  are  distributed.  In  other  words,  the  effect 
of  irritating  the  trunk  of  a  nerve  is  the  same  as  that  of 
irritating  its 'component  fibres  at  their  terminations.    • 

The  effects  just  described  will  follow  upon  irritation  of 
any -part  of  the  branches  of  the  nerve:  except  that  when 
a  branch  is  irritated  the  only  muscles  directly  affected,  and 
the  only  region  of  the  skin  to  which  pain  is  referred,  will 
be  those  to  which  that  branch  sends  nerve-fibres.  And 
these  effects  will  follow  upon  irritation  of  any  part  of  a 
nerve,  from  its  smallest  branches  up  to  the  point  of  its 
trunk,  at  which  the  anterior  and  posterior  root  fibres  unite. 

If  the  fibres  of  the  anterior  root  be  irritated  in  the 
same  way  (at_y,  Fig.  158),  only  half  the  previous  effects 
are  brought  about.  That  is  to  say,  all  the  muscles  to 
■  which  the  nerve  is  distributed  contract,  but  no  pain  is 
felt. 

So,  again,  if  the  posterior,  ganglionated  root  be  irritated 
(at  z,  Fig.  158),  only  half  the  effects  of  irritating  the  whole 
trunk  are  produced.  But  it  is  the  other  half;  that  is  to  say, 
none  of  the  muscles  to  which  the  nerve  is  distributed  con- 
tract, but  pain  is  referred  to  the  whole  area  of  skin  to  which 
the  fibres  of  the  nerve  are  distributed. 

It  is  clear  enough,  from  these  experiments,  that  all  the 
power  of  causing  muscular  contraction  which  a  spinal  nerve 
possesses  is  lodged  in  the  fibres  which  compose  its  anterior 
roots  ;  and  all  the  power  of  giving  rise  to  sensation,  in  those 
of  its  posterior  roots.  Hence  the  anterior  roots  are  com- 
monly called  motor,  and  the  posterior  sensory. 

The  same  truth  may  be  illustrated  in  other  ways.  Thus, 
if,  in  a  living  animal,  the  anterior  roots  of  a  spinal  nerve  be 
cut,  the  animal  loses  all  control  over  the  muscles  to  which 
that  nerve  is  distributed,  though  the  sensibility  of  the  region 


xii         FUNCTIONS   OF   ROOTS   OF   SPINAL   NERVES        497 

of  the  skin  supplied  by  the  nerve  is  perfect.  If  the  poste- 
rior roots  be  cut,  sensation  is  lost,  and  voluntary  movement 
remains.  But  if  both  roots  be  cut,  neither  voluntary  move- 
ment nor  sensibility  is  any  longer  possessed  by  the  part 
supplied  by  the  nerve.  The  muscles  are  said  to  be  para- 
lysed ;  and  the  skin  may  be  cut,  or  burnt,  without  any  sen- 
sation being  excited. 

If,  when  both  roots  are  cut,  that  end  of  the  motor  root 
which  remains  connected  with  the  trunk  of  the  nerve  be 
irritated,  the  muscles  contract ;  while,  if  the  other  end  be 
so  treated,  no  apparent  effect  results.     On  the  other  hand, 


■Ma^t  /-4^°* 


Fig.  158.  —  Diagram  to  illustrate  Experiments  in  Proof  of  the  Functions 
of  the  Spinal  Nerve  Roots  and  of  the  Ganglion  on  the  Posterior 
Root. 

AF,  anterior  fissure  of  spinal  cord;  PF,  posterior  fissure;  AR,  anterior  root  of 
spinal  nerve;  PR,  posterior  root;  T,  trunk  of  spinal  nerve;  Gti,  ganglion  of  pos- 
terior root. 

if  the  end  of  the  sensory  root  connected  with  the  trunk 
of  the  nerve  be  irritated,  no  apparent  effect  is  produced ; 
while,  if  the  end  connected  with  the  cord  be  irritated,  pain 
immediately  follows. 

When  no  apparent  effect  follows  upon  the  irritation  of 
any  nerve,  it  is  not  probable  that  the  molecules  of  the 
nerve  remain  unchanged.  On  the  contrary,  it  would  appear 
that  the  same  change  occurs  in  all  cases  ;  but  a  motor  nerve 
is  connected  with  nothing  that  can  make  that  change  appar- 
ent save  a  muscle,  and  a  sensory  nerve  with  nothing  that  can 
show  an  effect  but  the  central  nervous  system. 

2K 


498  ELEMENTARY   PHYSIOLOGY  less. 

It  is  an  interesting  fact  that  the  continued  life  of  any 
nerve-fibre  is  dependent  upon  the  continuance  of  its 
connection  with  the  cell  from  which  it  arises.  This  de- 
pendence is  shown  by  the  simple  experiment  of  cutting  a 
nerve,  and  preventing  the  cut  ends  from  reuniting.  Thus, 
if  the  anterior  (motor)  root  of  one  of  the  spinal  nerves  be 
cut  at  y  (Fig.  158),  all  the  fibres  of  that  root  beyond  y 
towards  and  along  the  trunk  of  the  nerve  T  degenerate. 
This  degeneration  shows  itself  by  structural  changes  in  the 
nerve-fibres,  which  result  ultimately  in  a  total  disappearance 
of  the  axis-cylinders  and  medullary  sheaths.  While  these 
structural  changes  are  taking  place,  and  even  before  they 
become  obvious,  the  irritability  of  the  nerve  becomes  gradu- 
ally less,  so  that  soon  the  nerve  makes  no  response  to  any 
stimulus  which  may  be  applied  to  it.  But  the  changes  we 
have  described  do  not  occur  in  that  (central)  part  of  the 
nerve  which  is  still  connected  with  the  cells  of  the  spinal 
cord  ;  the  portion  of  the  root  between  y  and  the  spinal  cord 
does  not  degenerate.  Hence  the  "  nutritional  centre,"  as 
it  may  be  called,  of  the  efferent  fibres  of  the  spinal  nerves 
lies  in  the  nerve-cell  bodies  of  the  anterior  cornu  of  the 
spinal  cord. 

If  we  apply  the  same  method  of  experiment  to  the 
posterior  root  the  following  results  are  observed :  when 
the  root  is  cut  at  w  (Fig.  158),  the  fibres  of  that  root 
towards  and  along  the  trunk  of  the  nerve  T  degenerate  ;  the 
central  parts  connected  with  the  ganglion  do  not.  If,  on 
the  other  hand,  the  posterior  root  is  cut  at  z,  then  the  part 
of  the  root  which  lies  between  z  and  the  spinal  cord  degen- 
erates, whereas  the  portion  still  connected  with  the  ganglion 
does  not.  Evidently  the  life  of  the  fibres  in  the  posterior 
root  is  dependent  upon  their  continued  connection  with  the 
cells   of  the   ganglion,  of  which  the   fibres  are   processes. 


xii  PHYSIOLOGICAL    PROPERTIES    OF   A   NERVE         499 

These  facts  lead  to  the  inevitable  conclusion  that  the  func- 
tion of  the  ganglion  of  the  posterior  root  is  to  provide  for 
the  nutrition  of  the  afferent  fibres  of  the  spinal  nerves. 

This  method  of  determining  and  localising  the  nutritional 
centres  from  which  nerve-fibres  grow  is  known  as  the  "  de- 
generation method,"  1  and  has  proved  to  be  most  helpful  in 
determining  the  various  "  tracts,"  or  paths  in  the  spinal  cord 
and  brain  along  which  nervous  impulses  of  various  kinds 
pass ;  with  these  we  shall  have  to  deal  later  on  (see  p. 
512). 

8.  The  Physiological  Properties  of  a  Nerve. — It  will 
be  observed  that  in  all  the  experiments  described  in  the 
first  part  of  the  preceding  section  there  is  evidence  that, 
when  a  nerve  is  irritated,  something  which  is  spoken  of  as 
a  nervous  impulse  and  consists,  probably,  of  a  change  in 
the  arrangement  of  its  molecules,  is  propagated  along  the 
nerve-fibres.  If  a  motor  or  a  sensory  nerve  be  irritated  at 
any  point,  contraction  in  the  muscle,  or  sensation  (or  some 
other  corresponding  event)  in  the  central  organ,  immedi- 
ately follows.  But  if  the  nerve  be  cut,  or  even  tightly  tied 
at  any  point  between  the  part  irritated  and  the  muscle  or 
central  organ,  the  effect  at  once  ceases,  just  as  cutting  a 
telegraph  wire  stops  the  transmission  of  the  electric  current 
or  impulse.  When  a  limb,  as  we  say,  "  goes  to  sleep,"  it  is 
frequently  because  the  nerves  supplying  it  have  been  sub- 
jected to  pressure  sufficient  to  interfere  with  the  nervous 
conductivity  of  the  fibres,  that  is,  their  power  to  transmit 
nervous  impulses.  We  lose  voluntary  control  over,  and  sen- 
sation in,  the  limb,  and  these  powers  are  only  gradually 
restored  as  that  nervous  conductivity  returns. 

Having  arrived  at  this  notion  of  an  impulse  travelling 

1  Also  as  the  "  Wallerian  method,"  after  the  name  of  the  physiologist 
who  first  employed  it. 


500  ELEMENTARY  PHYSIOLOGY  less. 

along  a  nerve,  we  readily  pass  to  the  conception  of  a  sen- 
sory nerve  as  a  nerve  which,  when  active,  brings  an  impulse 
to  the  central  organ,  or  is  afferent :  and  of  a  motor  nerve 
as  a  nerve  which  carries  away  an  impulse  from  the  organ,  or 
is  efferent.  It  is  more  convenient  to  use  these  terms  to 
denote  the  two  great  classes  of  nerves  than  the  terms  motor 
and  sensory ;  for  there  are  afferent  nerves  which  are  not 
sensory  in  the  sense  of  giving  rise  to  a  change  of  conscious- 
ness, or  sensation,  while  there  are  efferent  nerves  which  are 
not  motor,  in  the  sense  of  inducing  muscular  contraction. 
The  nerves,  for  example,  by  which  the  electrical  fishes  give 
rise  to  discharges  of  electricity  from  peculiar  organs  to 
which  those  nerves  are  distributed,  are  efferent,  inasmuch  as 
they  carry  impulses  to  the  electric  organs,  but  are  not  motor, 
inasmuch  as  they  do  not  give  rise  to  movements.  The 
pneumogastric  when  it  stops  the  beat  of  the  heart  cannot  be 
called  a  motor  nerve,  and  yet  is  then  acting  as  an  efferent 
nerve.  Similarly,  the  nerves  which  cause  the  cells  of  a 
gland  to  commence  secreting,  such  as  those  to  the  salivary 
glands,  sweat  glands,  pancreas,  etc.,  are  not  motor  nerves 
but  are  strictly  efferent  as  regards  the  direction  in  which 
they  convey  their  impulses.1  It  will,  of  course,  be  under- 
stood, as  pointed  out  above,  that  the  use  of  these  words 
does  not  imply  that  when  a  nerve  is  irritated  in  the  middle 
of  its  length  the  impulses  set  up  by  that  irritation  travel 
only  away  from  the  central  organ  if  the  nerve  be  efferent, 
and  towards  it  if  it  be  afferent.  On  the  contrary,  we  have 
evidence  that  in  both  cases  the  impulses  travel  both  ways. 
All  that  is  meant  is  this,  that  the  afferent  nerve  from  the  dis- 
position of  its  two  ends,  in  the  skin,  or  other  peripheral 

1  In  the  human  and  higher  vertebrate  body  it  is,  in  fact,  customary  to 
classify  efferent  nerves  into  *he  three  groups  of  motor,  inhibitory,  and 
secretory; 


xii  PHYSIOLOGICAL   PROPERTIES   OF   A   NERVE        501 

organs  on  the  one  hand,  and  in  the  cential  organ  on  the 
other,  is  of  use  only  when  impulses  are  travelling  along  it 
towards  the  central  organ,  and,  similarly,  the  efferent  nerve 
is  of  use  only  when  impulses  are  travelling  along  it  away 
from  the  central  organ. 

There  is  no  difference  in  structure,  in  chemical  or  in 
physical  character,  between  afferent  and  efferent  nerves. 
The  impulse  which  travels  along  them  requires  a  certain 
time  for  its  propagation,  and  is  vastly  slower  than  many 
other  movements  —  even  slower  than  sound.     (See  p.  504.) 

We  know  but  little  of  the  nature  of  a  nervous  impulse. 
We  know  that  it  may  be  started  in  a  nerve  by  various  artifi- 
cial means  such  as  by  pinching  or  knocking  the  nen  e,  or 
by  suddenly  warming  or  cooling  it,  and,  most  readily,  by 
stimulating  the  nerve  electrically.  And  we  suppose  that  by 
any  of  these  means  there  is  set  up  in  that  bit  of  nerve  to 
which  any  one  of  the  above  "  stimuli "  is  applied  a  disturb- 
ance, which  is  then  propagated  in  succession  from  one 
particle  (or  molecule)  of  the  axis-cylinder  to  the  next,  so 
that  it  ultimately  reaches  a  point  in  the  nerve  remote  from 
that  in  which  it  was  started.  In  this  way  we  come  to  speak 
of  a  nervous  impulse  as  due  to  the  propagation  of  a  "  molec- 
ular disturbance "  along  a  nerve.  But  this  expression 
serves  rather  to  hide  our  ignorance  than  to  explain  what  the 
impulse  really  is. 

If  we  may  illustrate  what  is  meant  by  this  expression,  by 
likening  the  process  of  the  transmission  of  a  nervous  impulse 
to  the  transmission  of  any  other  condition  with  which  most 
people  are  familiar,  we  might  compare  it  with  the  passage 
of  the  explosion  along  a  train  of  gunpowder  when  a  spark  is 
applied  to  one  end  of  it.  In  this  case  the  spark  merely  sets 
up  a  molecular  change  or  disturbance  in  the  grains  of  pow- 
der to  which  it  is  applied  ;  the  change  thus  set  up  leads  to 


502  ELEMENTARY   PHYSIOLOGY  less. 

a  similar  change  in  the  next  neighbouring  grains,  and  so  on 
along  the  whole  train  of  powder,  so  that  ultimately  the  result 
of  applying  the  spark  at  one  end  makes  its  appearance  as  a 
similar  result  at  the  other  end  of  the  train.  Similarly,  in  a 
nerve  we  may  regard  the  stimulus  as  setting  up  a  change, 
whose  nature  we  do  not  as  yet  understand,  at  the  point  to 
which  it  is  applied ;  this  change  sets  up  a  similar  change  in 
the  next  neighbouring  particles  of  the  nerve,  and  so  on  until 
it  finally  appears  at  the  furthest  end  of  the  nerve.  But  a 
nerve,  unlike  the  train  of  gunpowder,  relays  itself  so  long  as 
it  is  alive,  as  soon  as  the  impulse  has  passed  along  it,  whereas 
the  train  of  powder  is  "  dead  "  after  the  passage  of  the  explo- 
sion, and  must  be  artificially  relaid  for  further  use.  It  should 
be  borne  clearly  in  mind  that  the  simile  that  we  have  just 
used  is  to  be  taken  in  its  broadest  outlines  only,  and  that  we 
are  quite  ignorant  of  what  is  really  going  on  in  a  nerve  when 
in  action. 

The  Electrical  Properties  of  a  Nerve.  —  In  the  case  of  a  mus- 
cle we  saw  (p.  322)  that  its  entry  into  a  state  of  (contract- 
ing) activity  was  accompanied  by  an  easily  recognised  change 
of  shape,  by  chemical  changes  and  by  changes  of  tempera- 
ture. In  a  nerve,  when  it  is  active,  i.e.  is  conveying  an 
impulse,  the  first  of  these  changes  is  of  course  entirely  want- 
ing and  the  others  have  not  so  far  been  shown  to  take  place. 
But  we  saw  also  that  the  contracting  activity  of  a  muscle  is 
accompanied  by  an  electrical  disturbance  ;  a  similar  dis- 
turbance takes  place  in  a  nerve  as  the  impulse  sweeps  along 
it,  and  is,,  indeed,  the  only  evidence  we  possess  of  the  pas- 
sage of  the  impulse  at  any  moment. 

This  electrical  phenomenon  consists  in  the  fact  that  each 
successive  portion  of  the  nerve  becomes  electrically  negative 
as  the  impulse  passes  it.  This  electrical  change  sweeps  over 
the  nerve  in  the  form  of  a  wave.     It  may  readily  be  demon- 


xii  RATE   OF   A   NERVOUS    IMPULSE  503 

strated  in  an  excised  piece  of  nerve,  as,  for  instance,  the 
sciatic  nerve  of  a  frog  (see  Fig.  96),  by  connecting  two 
points  of  the  nerve  with  a  sensitive  galvanometer1  (Fig.  159) 
and  stimulating  at  some  other  point.  The  deflection  of  the 
needle  of  the  galvanometer  indicates  the  moment  when  the 
particular  portion  of  the  nerve  connected  with  the  galva- 
nometer goes  into  activity,  and  the  intensity  of  its  activity. 


Fig  159.  —  To  show  Arrangement  of  a  Nerve  and  Galvanometer  for  Ex- 
periments on  the  Electrical  Properties  of  a  Nerve. 

AB,  a  piece  of  nerve;  G,  a   galvanometer  connected  by  wires   and   the  electrodes 
a,  b,  with  the  end  B  and  the  middle  point  C  of  the  nerve. 

The  Rate  of  Transmission  of  a  Nervous  Impulse.  —  By  means 
of  a  complicated  arrangement  of  apparatus  it  is  possible  to 
determine  very  exactly  the  rate  at  which  the  electrical 
change  passes  over  the-  nerve,  and,  by  inference,  the  rate  of 
transmission  of  the  nervous  impulse.  This  is  found  to  be 
about  28  metres,  or  90  feet  per  second,  in  the  nerve  of  a  frog. 

The  rate  of  transmission  of  the  impulse  may  also  be 
determined  in  a  much  simpler  way,  by  using  a  muscle-nerve 
preparation  such  as  is  figured  on  page  321.  The  muscle  is 
suspended  from  a  clamp,  as  shown  in  Fig.  160 ;  a  light  hori- 
zontal lever  is  attached  by  a  hook  to  the  tendon  at  the  lower 
end  of  the  muscle,  so  that  when  the  muscle  is  made  to  con- 
tract the  free  end  of  the  lever  move's  upwards  and  thus  indi- 
cates the  moment  at  which  the  contraction  of  the  muscle 
commences.     The  sciatic  ..erve  is  then  arranged  in  such  a 

1  A  galvanometer  is  an  instrument  used  for  the  detection  and  measure- 
ment of  electric  currents. 


5G4  ELEMENTARY   PHYSIOLOGY  less 

way  that  it  may  be  stimulated  either  at  a  point  x  (Fig.  160, 
as  close  as  possible  to  its  junction  with  the  muscle,  or  at  a 
point  j'  as  far  away  as  possible  from  the  muscle.  By  the  use 
of  suitable  apparatus  it  is  easy  to  measure  the  interval  of 
time  which  elapses  between  the  moment  of  applying  the 
stimulus  at  x  and  the  moment  at  which  the  end  of  the  lever 
begins  to  move.  This  is  found  to  be,  in  an  ordinary  experi- 
ment, about  y^-g-  of  a  second.  If  now  the  nerve  is  stimu- 
lated at  y,  it  is  found  that  the  end  of  the  lever  begins  to 
move  slightly  later  than  it  did  when  the  stimulus  was  applied 
at  x ;  that  is  to  say,  the  muscle  begins  to  contract  rather 
later  when  its  nerve  is  stimulated  at  y  than  at  x.  This  dif- 
ference can  only  be  due  to  the  fact  that  when  the  impulse  is 
started  at  y  it  takes  longer  to  reach  the  muscle  than  when  it  is 
started  at  x.  Since  the  length  of  the  piece  of  nerve  between 
y  and  x  is  known  by  direct  measurement,  it  becomes  a  sim- 
ple matter  to  calculate  the  rate  at  which  the  impulse  travels 
from  y  to  x.  The  result  thus  obtained  agrees  quite  closely 
with  the  one  arrived  at  in  the  experiment  previously  de- 
scribed in  which  a  galvanometer  was  used,  namely  28  metres 
or  90  feet  per  second. 

The  rate  at  which  an  impulse  travels  along  a  nerve  is 
closely  dependent  on  the  temperature  of  the  nerve,  and 
diminishes  as  the  nerve  is  cooled  ;  thus,  by  cooling  a  frog's 
nerve  the  rate  may  be  reduced  to  as  little  as  1  metre  (3  feet) 
per  second.  Hence  it  is  not  surprising  that,  when  experi- 
ments are  made  on  the  nerves  of  a  warm-blooded  human 
being,  the  rate  of  transmission  is  found  to  be  somewhat 
greater,  viz.  33  metres  (or  rather  over  100  feet)  per  second, 
than  in  the  cold-blooded  frog. 

^The  idea  is  frequently  expressed  that  a  nervous  impulse  is 
of  the  nature  of  an  electric  current  similar  to  that  which 
passes  along  a  wire  as  used  for  telegraphy.     But  this  is  by 


XII 


RATE   OF  A   NERVOUS    IMTULSE 


5°5 


no  means  the  case,  since,  without  going  into  any  other  more 
abstruse  reasons,  we  have  shown  that  the  rate  at  which  an 
impulse  travels  along  a  nerve  is  on  an  average  about  33 
metres,  or  100  feet,  per  second,  whereas  we  know  that 
electricity  travels  along  a  wire  at  a  rate  such  that  the  trans- 
mission of  signals  over  the  wires  of  an  ordinary  land-line  is 
practically  instantaneous.  Even  in  one  of  the  cables  across 
the  Atlantic  Ocean  (2,500  miles  in  length)  only  two-tenths 
of  a  second  elapse  after  contact  is  made  with  the  battery  at 


»t> 


B" 


Fig.  160. — Arrangement  of  Nerve,  Muscle,  and  Lever  for  determining  the 
Velocity  of  a  Nervous  Impulse. 

J,  femur:  in,  gastrocnemius  muscle;  t  a,  tendon;  /,  lever  movable  about  the  end 
0;  to,  weight  to  keep  the  muscle  stretched;  «,  the  nerve;  x  and  y,  the  two  points  at 
which  the  nerve  is  stimulated. 


one  end  before  the  effect  can  be  first  detected  at  the  other 
end.  Now,  if  a  nerve  could  be  used  for  transmitting  an 
impulse  from,  say,  London  to  Liverpool  (200  miles),  the 
impulse  would  take  nearly  three  hours  (176  minutes)  to 
reach  its  destination,  travelling  as  it  does  at  the  rate  of  100 
feet  per  second. 


5o6  ELEMENTARY    PHYSIOLOGY  less. 

9.  The  Functions  of  the  Spinal  Cord.  —  Up  to  this  point 
our  experiments  have  been  confined  to  the  nerves.  We  may 
now  test  the  properties  of  the  spinal  cord  in  a  similar  way. 
If  the  cord  be  cut  across  (say  in  the  middle  of  the  back), 
the  legs  and  all  the  parts  supplied  by  nerves  which  come  off 
below  the  section  will  be  insensible,  and  no  effort  of  the  will 
can  make  them  move ;  while  all  the  parts  above  the  section 
will  retain  their  ordinary  powers. 

When  a  man  hurts  his  back  by  an  accident,  the  cord  is 
not  unfrequently  so  damaged  as  to  be  virtually  cut  in  two, 
and  then  insensibility  and  paralysis  of  the  lower  part  of  the 
body  ensue. 

If  when  the  cord  is  cut  across  in  an  animal  the  cut 
end  of  the  portion  below  the  division,  or  away  from  the 
brain,  be  irritated,  violent  movements  of  all  the  muscles  sup- 
plied by  nerves  given  off  from  the  lower  part  of  the  cord 
take  place,  but  no  sensation  is  felt  by  the  brain.  On  the 
other  hand,  if  that  part  of  the  cord  which  is  still  connected 
with  the  brain,  or  better,  if  any  afferent  nerve  connected 
with  that  part  of  the  cord  be  irritated,  sensations  ensue,  as  is 
shown  by  the  movements  of  the  animal ;  but  in  these  move- 
ments the  muscles  supplied  by  the  nerves  coming  from  the 
spinal  cord  below  the  cut  take  no  part;  they  remain  per- 
fectly quiet. 

Thus,  it  may  be  said  that,  in  relation  to  the  brain,  the 
cord  is  a  great  mixed  motor  and  sensory  nerve.  But  it  is 
also  much  more. 

Reflex  Action  through  the  Spinal  Cord.  —  If  the  trunk  of  a 
spinal  nerve  be  cut  through,  so  as  to  sever  its  connection 
with  the  cord,  an  irritation  of  the  skin  to  which  the  sensory 
fibres  of  that  nerve  are  distributed  produces  neither  motor 
nor  sensory  effect.  But  if  the  cord  be  cut  through  any- 
where so  as  to  sever  its  connection  with  the  brain,  irrita- 


xii  THE   FUNCTIONS   OF  THE   SPINAL  CORD  507 

tiun  applied  to  the  skin  of  the  parts  supplied  with  sensory 
nerves  from  the  part  of  the  cord  below  the  section,  though 
it  gives  rise  to  no  sensation,  may  produce  violent  motion  of 
the  parts  supplied  with  motor  nerves  from  the  same  part 
of  the  cord. 

Thus,  in  the  case  supposed  above,  of  a  man  whose  legs 
are  paralysed  and  insensible  from  spinal  injury,  tickling 
the  soles  of  the  feet  will  cause  the  legs  to  kick  out  con- 
vulsively. And  as  a  broad  fact,  it  may  be  said  that,  so 
long  as  both  roots  of  the  spinal  nerves  remain  connected 
with  the  cord,  irritation  of  any  afferent  nerve  is  competent 
to  give  rise  to  excitement  of  some,  or  the  whole,  of  the 
efferent  nerves  so  connected. 

If  the  cord  be  cut  across  a  second  time  at  any  distance 
below  the  first  section,  the  efferent  nerves  below  the  second 
cut  will  be  affected  no  longer  by  irritation  of  the  afferent 
nerves  above  it,  but  only  by  irritation  of  those  below  the 
second  section.  Or,  in  other  words,  in  order  that  an  affe- 
rent impulse  may  be  converted  into  an  efferent  one  by  the 
spinal  cord,  the  afferent  nerve  must  be  in  uninterrupted 
material  communication  with  the  efferent  nerve  by  means 
of  the  substance  of  the  spinal  cord. 

This  peculiar  power  of  the  cord,  by  which  it  is  compe- 
tent to  convert  afferent  into  efferent  impulses,  is  that  which 
distinguishes  it  physiologically,  as  a  central  organ,  from  a 
nerve,  and  is  called  reflex  action.  It  is  a  power  possessed 
by  the  grey  matter,  and  not  by  the  white  substance  of  the 
cord. 

The  number  of  the  efferent  nerves  which  may  be  excited 
by  the  reflex  action  of  the  cord  is  not  regulated  alone  by 
the  number  of  the  afferent  nerves  which  are  stimulated  by 
the  irritation  which  gives  rise  to  the  reflex  action.  Nor 
does  a  simple  excitation  of  the  afferent  nerve  by  any  means 


508  ELEMENTARY   PHYSIOLOGY  less 

necessarily  imply  a  corresponding  simplicity  in  the  arrange- 
ment and  succession  of  the  reflected  motor  impulses.  Tick- 
ling the  sole  of  the  foot  is  a  very  simple  excitation  of  the 
afferent  fibres  of  its  nerves  ;  but  in  order  to  produce  the 
muscular  actions  by  which  the  legs  are  drawn  up,  a  great 
multitude  of  efferent  fibres  must  act  in  regulated  combina- 
tion. In  fact,  in  a  multitude  of  cases  a  reflex  action  is  to 
be  regarded  rather  as  the  result  of  a  dormant  activity  of 
the  spinal  cord  awakened  by  the  arrival  of  the  afferent  im- 
pulse, as  a  sort  of  orderly  explosion  fired  off  by  the  afferent 
impulse,  than  as  a  mere  rebound  of  the  afferent  impulse 
into  the  first  efferent  channels. open  to  it. 

The  various  characters  of  these  reflex  actions  may  be 
very  conveniently  studied  in  the  frog.  If  a  frog  be  decapi- 
tated, or,  better  still,  if  the  spinal  cord  be  divided  close  to 
the  head,  and  the  brain  be  destroyed  by  passing  a  blunt 
wire  into  the  cavity  of  the  skull,  the  animal  is  thus  de- 
prived (by  an  operation  which,  being  almost  instantaneous, 
can  give  rise  to  very  little  pain)  of  all  consciousness  and 
volition,  and  yet  the  spinal  cord  is  left  intact.  At  first  the 
animal  is  quite  flaccid  and  apparently  dead,  no  movement 
of  any  part  of  the  body  (except  the  beating  of  the  heart) 
being  visible.  This  condition,  however,  being  the  result 
merely  of  the  so-called  shock  of  the  operation,  very  soon 
passes  off,  and  then  the  following  facts  may  be  observed  : 

So  long  as  the  animal  is  untouched,  so  long  as  no  stimu- 
lus is  brought  to  bear  upon  it,  no  movement  of  any  kind 
takes  place  :  volition  is  wholly  absent. 

If,  however,  one  of  the  toes  be  gently  pinched,  the  leg 
is  immediately  drawn  up  close  to  the  body. 

If  the  skin  between  the  thighs  around  the  anus  be 
pinched,  the  legs  are  suddenly  drawn  up  and  thrust  out 
again  violently. 


Xu  THE   FUNCTIONS   OF  THE   SPINAL  CORD  509 

If  the  flank  be  very  gently  stroked,  there  is  simply  a 
twitching  movement  of  the  muscles  underneath  ;  if  it  be 
more  roughly  touched,  or  pinched,  these  twitching  move- 
ments become  more  general  along  the  whole  side  of  the 
creature,  and  extend  to  the  other  side,  to  the  hind  legs, 
and  even  to  the  front  legs. 

If  the  digits  of  the  front  limbs  be  touched,  these  will  be 
drawn  close  under  the  body  as  in  the  act  of  clasping. 

If  a  drop  of  vinegar  or  any  acid  be  placed  on  the  top 
of  one  thigh,  rapid  and  active  movements  will  take  place 
in  the  leg.  The  foot  will  be  seen  distinctly  trying  to  rub 
off  the  drop  of  acid  from  the  thigh.  And  what  is  still 
more  striking,  if  the  leg  be  held  tight  and  so  prevented 
from  moving,  the  other  leg  will  begin  to  rub  off  the  acid. 
Sometimes,  if  the  drop  be  too  large  or  too  strong,  both 
legs  begin  at  once,  and  then  frequently  the  movements 
spread  from  the  legs  all  over  the  body,  and  the  whole 
animal  is  thrown  into  convulsions. 

Now  all  these  various  movements,  even  the  feeblest  and 
simplest,  require  a  certain  combination  of  muscles,  and  some 
of  them,  such  as  the  act  of  rubbing  off  the  acid,  are  in  the 
highest  degree  complex.  In  all  of  them,  too,  a  certain  pur- 
pose or  end  is  evident,  which  is  generally  either  to  remove 
the  body,  or  part  of  the  body,  from  the  stimulus,  from  the 
cause  of  irritation,  or  to  thrust  away  the  offending  object 
from  the  body  :  in  the  more  complex  movements  such  a 
purpose  is  strikingly  apparent. 

It  seems,  in  fact,  that  in  the  frog's  spinal  cord  there  are 
sets  of  nervous  machinery  destined  to  be  used  for  a  variety 
of  movements,  and  that  a  stimulus  passing  along  a  sensory 
nerve  to  the  cord  sets  one  or  the  other  of  these  pieces  of 
machinery  at  work. 

Thus,  one  important    function  of  the  spinal  cord  is  to 


5i°  ELEMENTARY    PHYSIOLOGY  less. 

serve  as  an  independent  nervous  centre,  capable  of  origi- 
nating combined  movements  upon  the  reception  of  the  im- 
pulse of  an  afferent  nerve,  or  rather,  perhaps,  a  group  of 
such  independent  nervous  centres. 

In  all  these  reflex  actions  of  the  spinal  cord,  the  struc- 
tures necessary  for  their  performance  are,  as  already  pointed 
out  (p.  369),  a  sensory  surface,  an  afferent  nerve,  a  portion 
of  the  grey  matter  of  the  cord,  an  efferent  nerve,  and  a 
muscle  or  group  of  muscles  (Fig.  150,  B).  In  the  case 
of  the  headless  frog,  the  actions  are  of  course  quite  invol- 
untary, and  performed  unconsciously,  and  the  same  remark 
holds  good  in  the  case  of  a  man  whose  spinal  cord  is  so 
injured  as  to  be  practically  cut  in  two.  But  even  in  an 
uninjured,  healthy  man,  similar  reflex  actions,  although  now 
under  the  control  of  the  will,  are  strikingly  manifest,  and 
play  an  important  part  in  his  everyday  life.  Thus,  the  act 
of  walking,  though  started  by  the  will,  is  subsequently  a 
reflex  action.  When  engaged  in  conversation  or  buried  in 
thought,  a  person  walks  with  all  his  ordinary  dexterity,  but 
in  entire  unconsciousness  of  the  action.  In  this  case  the 
afferent  impulses  are  largely  started  from  the  stimulation  of 
the  skin  of  the  feet  and  legs  which  results  from  the  varying 
pressure  and  contact  with  the  ground.  Hence  the  stagger- 
ing gait  in  cases  where,  as  a  result  of  disease,  the  chain 
of  structures  requisite  for  the  liberation  of  the  reflexes  is 
broken,  as  for  instance  by  disease  of  the  posterior  (affe- 
rent) roots  of  the  spinal  cord.  In  such  cases  walking  is  fre- 
quently possible  only  as  the  result  of  looking  at  the  ground; 
this  accords  with  the  fact  that  even  in  health  afferent  im- 
pulses started  in  the  sensory  surface  (retina)  of  the  eye 
play  an  important  part  in  giving  rise  to  the  reflexes  of 
walking.  But,  on  the  other  hand,  blind  persons  walk  with 
no  little  dexterity,  using  other  sensory  impulses. 


xii  THE   FUNCTIONS   OF   THE   SPINAL   CORD  511 

Again,  the  actions  of  micturition  and  defalcation  are  really 
reflex  actions  carried  out  by  the  spinal  cord  as  soon  as  they 
have  been  started  by  the  will ;  here  the  sensory  surfaces  are 
the  mucous  membrane  of  the  bladder  or  rectum,  the  neces- 
sary stimulus  being  supplied  as  the  result  of  their  distension 
by  the  accumulated  urine  or  faeces. 

Using  the  expression  reflex  action  in  a  rather  wider  and 
more  general  sense  we  may  here  again  draw  attention  to 
the  importance  of  these  actions  to  the  working  and  welfare 
of  the  body  as  regards  the  relationships  of  its  internal 
mechanisms.  Thus,  we  have  seen  that  certain  parts  of  the 
spinal  bulb,  or  medulla,  are  connected  with  the  heart  (cardio- 
inhibitory  centre),  blood-vessels  (vaso-motor  centre),  and 
respiratory  muscles  (respiratory  centre),  in  such  a  way  that 
impulses  arising  in  outlying  parts  of  the  body  lead  reflexly 
to  such  modified  activity  of  each  of  the  above  systems  as  may 
from  time  to  time  be  necessary  (see  pages  95,  101,  180). 

Reflex  action  is  a  property  of  the  central  nervous  system 
which  is  not  confined  to  the  spinal  cord  alone,  or  to  the 
spinal  bulb  to  which  we  have  just  extended  it,  but  is  also  a 
marked  characteristic  of  the  varied  activities  of  the  brain. 
But  to  this  point  we  shall  return  later  on. 

The  Paths  of  Conduction  of  Impulses  along  the  Spinal  Cord. 
—  The  spinal  cord  has  a  further  most  important  function 
beyond  reflex  action,  namely  that  of  transmitting  nervous 
impulses,  as  a  great  mixed  motor  and  sensory  nerve  leading 
from  the  brain,  between  the  brain  and  the  various  organs, 
such  as  the  muscles  and  the  skin,  with  which  the  spinal 
nerves  are  connected.  When  we  move  a  foot,  certain 
nervous  impulses,  starting  in  some  part  of  the  cerebral  hemi- 
spheres, pass  down  along  the  whole  length  of  the  spinal 
cord  as  far  as  the  roots  of  the  spinal  nerves  going  to  the 
legs,  and  issuing  along  the  fibres  of  the  anterior  bundles  of 


512  ELEMENTARY   PHYSIOLOGY  less. 

these  roots  find  their  way  to  the  muscles  which  move  the 
foot.  Similarly,  when  the  sole  of  the  foot  is  touched,  affe- 
rent impulses  travel  in  the  reverse  way  upwards  along  the 
spinal  cord  to  the  brain.  And  the  question  arises,  in  what 
manner  do  these  efferent  and  afferent  impulses  travel  along 
the  spinal  cord? 

This  question  is  one  very  difficult  to  answer,  and  indeed 
a  complete  and  exact  statement  is  not,  at  present,  possible. 
The  method  by  which  a  large  amount  of  our  present  infor- 
mation on  this  matter  has  been  obtained  is  the  degeneration 
method  already  described  (p.  498).  As  in  the  case  of  a 
nerve,  so,  if  the  spinal  cord  be  cut  across,  degeneration 
changes  of  the  fibres  of  the  white  matter  start  from  the 
place  of  the  cut,  and  advance  upwards  and  downwards  along 
the  cord.  These  changes  affect  certain  definite  areas, 
which  differ  above  and  below  the  cut.  Degeneration  up- 
wards is  known  as  ascending  degeneration :  degeneration 
downwards,  as  descending  degeneration.  The  areas  or  tracts 
thus  affected  represent  paths  of  conduction  along  the  cord. 
In  general,  within  the  cord,  fibres  degenerate  in  the  same 
direction  in  which  they  conduct,  hence  from  the  direction 
of  degeneration  the  direction  of  conduction  may  be 
inferred. 

The  chief  tracts  of  ascending  degeneration  and  conduc- 
tion are  shown  in  Fig.  161,  and  those  of  descending  degen- 
eration and  conduction  in  Fig.  162.  It  will  be  seen  that 
the  majority  of  the  afferent  (sensory)  impulses  on  their  way 
to  the  brain  pass  up  the  cord  in  the  posterior  and  lateral 
columns,  the  postero-median  tracts  {p.m.)  being  the  chief 
path  to  the  cortex  of  the  cerebrum  (see  pp.  518,  529),  the 
two  others  that  are  figured  going  to  the  cerebellum  (see 
pp.  518,  530).  The  efferent  (motor)  impulses,  however, 
come  down  the  cord  from  the  brain  mainly  in  the  anterior 


XII  THE    FUNCTIONS   OF   THE    SPINAL   CORD  513 

and  lateral  portions,  the  crossed  pyramidal  tract  ( Cr.p.) 
being  the  most  conspicuous  column  and  conveying  the  im- 
pulses directly  down  from  the  cortex  of  the  cerebrum. 

Besides  these  tracts  for  ascending  and  descending  con- 
duction, there  is  constant  intercommunication  going  on 
between  the  two  sides  of  the  cord.  This  is  made  possible 
by  fibres  which  cross  from  one  side  to  the  other  in  the 
bridge  connecting  the  two  halves  (see  Fig.  148,4  and  5). 


Cb. 


<s0~~-asc.  a.L. 


A.F. 


Fig  161. — Diagram  to  show  the  Position  of  Tracts  of  Ascending  De- 
generation in  the  White  Matter  of  the  Spinal  Cord  at  the  Level  of 
the  Fifth  Cervical  Nerve. 

A.F.,  anterior  fissure:  P.F.,  posterior  fissure;  /  in.,  p.m.,  the  postero-median 
tract,  or  tract  of  fibres  from  the  posterior  roots  of  the  spinal  nerves;  Cb.,  Cb.,  the 
direct  cerebellar  tract;  asc.a.l.,  asc.a.l.,  the  ascending  antero-lateral  tract. 

The  grey  matter  of  the  cord  is  shaded  black. 


Such  are  the  functions  of  the  spinal  cord;  taken  as  a 
whole.  The  spinal  nerves  are,  as  we  have  said,  chiefly  dis- 
tributed to  the  muscles  and  to  the  skin.  But  other  nerves, 
such  as  those,  for  instance,  belonging  to  the  blood-vessels, 
the  vaso-motor  nerves  (p.  90),  though  many  of  them  run 
for  long  distances  in  the  sympathetic  system,  may  ultimately 
be  traced  to  the  spinal  cord.  Along  the  spinal  column  the 
spinal  nerves  give  off  branches  which  run  into  and  join  the 
sympathetic  system.     And  the  vaso-motor  fibres  which  run 

2L 


5J4 


ELEMENTARY   PHYSIOLOGY 


along  in  the  sympathetic  nerves  do  really  spring  from  the 
spinal  cord,  finding  their  way  into  the  sympathetic  system 
through  these  communicating  or  commissural  branches. 
Besides  these,  some  vaso-motor  fibres  run  in  spinal  nerves 
along  their  whole  course. 

The  cord  is,  therefore,  spoken  of  as  containing  centres  for 
the  vaso-motor  nerves  or,  more  shortly,  vaso-motor  centres. 
Irritation  of  particular  regions  of  the  cord  produces  the 
same  effect  as  irritation  of  the  vaso-motor  nerves  themselves, 
and  destruction  of  those  parts  of  the  cord  paralyses  the 
vaso-motor  nerves. 

P.F. 


desc.a.L. 


D.'p'''   A.F.    ~~-Dp. 


Fig.  162. —  Diagram  to  show  the  Position  of  Tracts  of  Descending  De- 
generation in  the  White  Matter  of  the  Spinal  Cord  at  the  same 
Level  as  in  Fig.  161. 

Cr.p.,   Cr'.p'.,  crossed   pyramidal   tracts;    D'.p'.,   D.p..    direct   pyramidal   tracts; 
desc.  a. I.,  desc.  ad.,  descending  antero-lateral  tract. 

It  will,  however,  be  remembered  that  the  nervous  influ- 
ence does  not  originate  here,  but  proceeds  from  higher  up, 
from  the  chief  vaso-motor  centre  in  the  medulla  oblongata, 
in  fact,  and  simply  passes  down  through  this  part  of  the 
spinal  cord  on  its  way  to  join  the  sympathetic  nerves. 

10.  The  Sympathetic  Nervous  System.  —  The  sympa- 
thetic system  consists  chiefly  of  a  double  chain  of  ganglia 
lying  at  the  sides  and  in  front  of  the  spinal  column,  and  con- 
nected with  one   another,  and  with  the   spinal  nerves,  by 


xii  THE   SYMPATHETIC   NERVOUS   SYSTEM  515 

commissural  cords  (Fig.  147).  From  these  ganglia,  nerves 
are  given  off  which  for  the  most  part  follow  the  distribution 
of  the  blood-vessels,  but  which,  in  the  thorax  and  abdomen, 
form  great  networks,  ox  plexuses,  upon  the  heart  and  about 
the  stomach  and  other  abdominal  viscera.  A  great  number 
of  the  fibres  of  the  sympathetic  system  are  derived  from  the 
spinal  cord  with  the  spinal  nerves,  but  others  originate  in 
the  ganglia  of  the  sympathetic  itself;  some  run  back  into 
the  spinal  nerves  for  distribution  to  the  blood-vessels  of  the 
limbs. 

By  means  of  the  sympathetic  nerves  the  muscles  of  the 
vessels  generally,  and  those  of  the  heart,  of  the  intestines, 
and  of  some  other  viscera  may,  as  we  have  seen,  be  influ- 
enced ;  and  the  influence  thus  conveyed,  it  may  be  remarked, 
is  generally  different  to,  or  even  antagonistic  to  that  which 
is  conveyed  to  the  same  organs  by  the  fibres  running  in  the 
spinal  or  cranial  nerves.  Thus,  while  irritation  of  the 
(cranial)  pneumogastric  fibres  slows  or  stops  the  heart,  irri- 
tation of  the  sympathetic  fibres  going  to  the  heart  quickens 
the  beat. 

But  the  influences  which  thus  reach  these  organs  through 
the  sympathetic  nerves  do  not  originate  in  the  sympathetic 
system  itself,  but  are  derived  from  the  spinal  cord  or  brain. 
We  have  seen  (p.  94)  this  to  be  the  case  in  reference  to 
vaso-motor  nerves  and  the  same  is  true  of  the  sympathetic 
nerves  going  to  the  heart  and  other  viscera.  Whatever  may 
turn  out  to  be  the  function  of  the  sympathetic  ganglia,  there 
is  at  present  no  adequate  evidence  that  they  in  any  way  act 
as  nervous  centres,  either  of  reflex  action,  or  of  any  other 
form  of  nervous  activity.  Hence  the  sympathetic  is  not  to 
be  regarded  as  a  separate  nervous  system,  but  as  being  in 
reality  merely  an  outlying  part  of  the  cerebro-spinal  system, 
an  outlying  chain  of  ganglia,  through  which  the  fibres  of  a 


Si6 


ELEMENTARY   PHYSIOLOGY 


LESa 


part  of  the  trunk  of  each  spinal  nerve  pass  on  their  way  to 
the  viscera.  This  relationship  is  made  quite  clear  by  the 
accompanying  diagram  (Fig.  163). 

The  ganglia  of  the  sympathetic  system  are  composed  of 
nerve-cells  bound  together  by  a  small  amount  of  loose  con- 
nective tissue.    The  cells  differ  somewhat  in  appearance  and 


Fig.   163.  —  Diagram  to  illustrate  the  Distribution  of  the  Spinal  Nerves 
and  their  Relationship  to  the  Ganglia  of  the  Sympathetic  System. 

A.F,  anterior  fissure;  P.F,  posterior  fissure;  Gr,  grey  matter,  IV,  white  matter 
of  spinal  cord;  A,  anterior  root  of  spinal  nerve:  Gu,  ganglion  on  the  posterior  root; 
N,  the  trunk  of  a  spinal  nerve:  N' ,  spinal  nerve  proper,  ending  in  a  skeletal  muscle 
M,  in  a  sensory  cell  or  surface  S,  or  in  other  ways  X\  V,  a  branch  (white  ramus 
communicans)  of  the  spinal  nerve  passing  to  2,  a  ganglion  of  the  sympathetic  system, 
then  passing  on  as  V  to  some  more  distant  ganglion  a  and  then  as  V"  to  some 
peripheral  ganglion  er',  and  ending  in  a  muscle  m  of  the  blood-vessels  or  viscera,  in 
s,  an  internal  (visceral)  sensory  cell  or  surface,  or  in  other  ways  x. 

From  2  a  nerve  r.v  (grey  ramus  communicans)  runs  back  and  passes  partly 
towards  the  spinal  cord  and  partly,  as  vaso-motor  fibres  v.m,  in  connection  with  the 
spinal  nerve  N' ,  to  m'  the  muscles  of  blood-vessels  in  certain  parts  e.g.  of  the  limbs. 

Sy,  Sy,  the  main  chain  of  the  sympathetic  system,  which  unites  the  several  gan- 
glia, 2,  of  that  system.     (See  Fig.  147.) 


arrangement  according  to  the  ganglion  in  which  they  are 
seen,  but,  speaking  broadly  and  generally,  they  may  be  said 
to  resemble  in  their  most  obvious  features  the  motor  cells  of 
the  spinal  cord,  whose  structure  we  have  previously  de- 
scribed (p.  491).  Like  the  latter,  each  nerve-cell  of  a 
sympathetic  ganglion  contains  a  large  and  conspicuous 
nucleus,  and  the  cell-body  is  prolonged  into  a  varying  but 
usually  large  number  of  branching  processes   (dendrites). 


xii  THE  ANATOMY   OF  THE   BRAIN  517 

Moreover,  each  cell  possesses  one  process  which  does  not 
branch  but  passes  away  from  the  body  of  the  cell  as  a 
(usually)  non-medullated  nerve-fibre,  to  be  distributed  to 
the  various  tissues  (p.  515)  which  it  influences,  and  is  in 
this  respect  similar  to  the  axis-cylinder  process  of  a  cell  of 
the  spinal  cord.  Each  ganglion  is  also  connected  with 
nerve -fibres  which  come  to  it  from  the  central  nervous 
system  (Fig.  163).  These  fibres  mostly  end  in  connections 
with  the  nerve-cells.  Sometimes  a  fibre  does  not  end  in 
the  first  ganglion  it  meets,  but  passes  right  through  it  into 
one  of  the  nerves  going  off  from  that  ganglion,  and  so 
reaches  some  other  more  distant  ganglion.  The  number  of 
nerve-fibres  which  thus  pass  into  the  ganglion  from  the 
spinal  cord  is  much  less  than  the  number  of  nerve-cells  in 
the  ganglion  ;  hence  many  more  nerve-fibres  are  found  com- 
ing from  the  ganglion  than  entering  it.  By  this  arrange- 
ment each  ganglion  provides,  as  it  were,  a  sort  of  junction 
by  means  of  which  any  nervous  impulses  which  reach  it 
along  any  one  path  may  be  the  more  readily  and  widely  dis- 
tributed, along  several  paths,  to  the  tissues. 

11.  The  Anatomy  of  the  Brain. — The  brain  is  a  very 
complex  organ,  consisting  of  many  parts.  It  occupies  the 
cavity  of  the  skull  and  is  thus  placed  at  the  upper  end  of 
the  spinal  cord  with  which  it  becomes  connected  by  means 
of  the  spinal  bulb ;  1  this  passes  insensibly  into,  and  in  its 
lower  part  has  the  same  structure  as,  the  spinal  cord.  When 
viewed  from  the  side,  after  the  removal  of  the  bones  of  the 
skull,  the  brain  presents  the  appearance  shown  in  the  follow- 
ing figure  (Fig.  164).  The  spinal  cord  (/V)  widens  out 
into  the  bulb  (M.Ob?)  ;  this  is  continued  by  other  struc- 
tures not  clearly  visible  in  the  figure  into  the  large  convoluted 

1  Throughout  this  section  we  shall  use  the  word  "  bulb  "  for  "  spinal 
bulb,"  and  instead  of  the  older  name  "  medulla  oblongata." 


5'8 


ELEMENTARY  PHYSIOLOGY 


structure  (C,  C,  C)  which  is  called  the  cerebrum,  consisting 
of  a  right  and  a  left  cerebral  hemisphere.  Lying  beneath  the 
hinder  end  of  the  hemispheres  is  a  large   laminated  mass 


Fig.  164.  —  Side  View  of  the  Brain  and  Upper  Part  of  the  Spinal  Cord  in 
Place  —  the  Parts  which  cover  them  being  removed. 

C,C,  the  convoluted  surface  of  the  right  cerebral  hemisphere;  Cfi,  the  cerebellum; 
M.  Ob.,  the  medulla  oolongata;  B,  the  bodies  of  the  cervical  vertebrae;  Sfi,  their  spines; 
/V,  the  spinal  cord  with  the  spinal  nerves. 

which  overhangs  the  posterior  side  of  the  bulb  and  is  known 
as  the  cerebellum  (Cb.).  When  the  brain  is  removed  from 
the  skull  and  looked  at  from  its  base  or  under-surface,  many 


xii  THE   ANATOMY   OF  THE   BRAIN  519 

further  details  may  be  at  once  made  out.  Thus,  it  becomes 
evident  that  the  brain  consists  of  two  halves,  corresponding 
to  each  half  of  the  spinal  cord,  lying  symmetrically  on  each 
side  of  a  line  joining  CC  and  M  in  Fig.  165.  The  bulb 
(M)  widens  out  at  its  upper  end  and  gives  off  from  each 
side  a  number  of  nerves  (  VII -XIIs),  which  are  analogous  to 
the  spinal  nerves,  but,  as  originating  from  the  brain,  are 
called  cranial  nerves.  The  other  six  pairs  of  cranial  nerves 
(I -VI)  come  off  from  parts  of  the  brain  in  front  of  (above) 
the  bulb.  The  cerebellum  is  seen  to  send  out  from  each 
side  towards  the  central  line  a  large  mass  of  transverse  fibres 
which  sweep  across  the  brain  and  meet,  with  a  depression  in 
the  middle  line,  thus  forming  a  sort  of  bridge  from  one-half 
of  the  cerebellum  to  the  other ;  this  bridge  lies  just  in  front 
of  (above)  the  bulb  and  is  called  the  pons  Varolii.  The 
number  VI  is  placed  upon  the  pons  in  Fig.  165.  The  bulb, 
pons,  and  cerebellum  together  constitute  what  is  often  called 
the  hind-brain.  The  longitudinal  nerve-fibres  of  the  bulb 
pass  forwards  (upwards  in  the  figure),  among  and  between 
the  transverse  fibres  of  the  pons,  and  become  visible  again 
in  front  of  it  as  two  broad  diverging  bundles  called  crura 
cerebri,  which  plunge  into  the  corresponding  cerebral  hemi- 
sphere of  each  half  of  the  brain.  The  crura  cerebri  with  the 
parts  lying  directly  upon  them  (the  so-called  corpora  quadri- 
gemina,  see  below)  constitute  the  mid-brain.  The  cerebral 
hemispheres,  with  what  is  included  between  them,  form  the 
fore-brain.  These  terms,  fore-brain,  mid-brain,  and  hind- 
brain,  are  more  plainly  applicable  in  the  lower  vertebrates, 
such  as  fishes,  amphibians,  and  reptiles,  where  the  three 
chief  parts  of  the  brain  are  arranged  in  a  straight  line,  and 
do  not  overlap  one  another  to  a  great  extent. 

When  the  brain  is  viewed  from  above  nothing  is  visible 
beyond  the  convoluted  surfaces  of  the  two  cerebral  hemi- 


5*> 


ELEMENTARY   PHYSIOLOGY 

cp 


Fig.  165.  —  The  Base  or  Under-surface  of  the  Brain. 


A,  frontal  lobe;  B,  temporal  lobe  of  the  cerebral  hemispheres;  Cb,  cerebellum; 
/,  the  olfactory  lobe;  //,  the  optic  nerve;  ///,  IV,  VI,  the  nerves  of  the  muscles  of 
the  eye;  V,  the  trigeminal  nerve;  VII,  the  facial  nerve;  VIII,  the  auditory  nerve; 
IX,  the  glossopharyngeal ;  X,  the  pneumogastric;  XI,  the  spinal  accessory;  XII, 
the  hypoglossal,  or  motor  nerve  of  the  tongue.  The  number  J  Yis  placed  upon  the 
pons  Varolii.  The  crura  cerebri  are  the  broad  bundle  of  fibres  which  lie  between 
the  third  and  the  fourth  nerves  on  each  side.  The  medulla  oblongata  (M)  is  seen  to  be 
really  a  continuation  of  the  spinal  cord;  on  the  lower  end  are  seen  the  two  crescents 
of  grey  matter;  the  section,  in  fact,  has  been  carried  through  the  spinal  cord,  a  little 
below  the  proper  medulla  oblongata.  From  the  sides  of  the  medulla  oblongata  are 
seen  coming  off  the  X,  XI,  and  XII  nerves;  and  just  where  the  medulla  is  covered, 
so  to  speak,  by  the  transversely  disposed  pons  Varolii,  are  seen  coming  off  the  VII 
nerve,  and  more  towards  the  middle  line,  the  VI.  Out  of  the  substance  of  the  pons 
springs  the  V  nerve.  In  front  of  that  is  seen  the  well-defined  anterior  border  of  the 
pons;  and  coming  forward  in  front  of  that  line,  between  the  IV  and  ///nerves  on 
either  side,  are  seen  the  crura  cerebri.  The  two  round  bodies  in  the  angle  between 
the  diverging  crura  are  the  so-called  corpora  albicantia,  and  in  front  of  them  is  P, 
the  pituitary  body.  This  rests  on  the  chiasma,  or  junction,  of  the  optic  nerves;  the 
continuation  of  each  nerve  is  seen  sweeping  round  the  crura  cerebri  on  either  side. 
Immediately  in  front,  between  the  separated  frontal  lobes  of  the  cerebral  hemispheres, 
is  seen  the  corpus  callosurn,  CC.  The  fissure  of  Sylvius,  about  on  a  level  with  /on 
the  left  and  //  on  the  right  side,  marks  the  division  between  frontal  and  temporal  lobes. 


Xii  THE   ANATOMY   OF   THE    BRAIN  521 

spheres,  separated  by  a  median  fissure  whose  sides  are  in 
close  contact.  But  if  the  sides  of  this  fissure  are  carefully 
pushed  apart,  the  cerebral  hemispheres  may  be  seen  to  be 
connected  with  each  other  by  an  elongated  transverse  and 
horizontal  mass  of  nerve-fibres  known  as  the  corpus  callo- 
sum  (shown  as  CC  in  Fig.  165).  If  the  hinder  ends  of 
the  cerebral  hemispheres  are  raised,  the  whole  upper  surface 
of  the  cerebellum  comes  into  view,  and  if  the  cerebellum  is 
now  lifted  up,  the  posterior  surface  of  the  bulb  is  exposed. 
Unlike  the  anterior  surface,  which  is  conspicuously  convex 
(see  Fig.  165,  M~),  the  posterior  surface  of  the  bulb  is  marked 
by  a  shallow,  elongated,  diamond-shaped  depression,  forming 
the  cavity  of  the  fourth  ventricle.  This  cavity  arises  from 
the  gradual  divergence  of  the  posterior  white  columns  of  the 
spinal  cord,  while  the  depth  of  the  posterior  fissure  is  at  the 
same  time  diminished,  so  that  the  central  canal  of  the  spinal 
cord  approaches  the  floor  of  the  fourth  ventricle,  and  act- 
ually opens  into  the  lower  end  of  the  cavity  (Fig.  166)  ; 
this  lower  end  of  the  ventricle  is  known  as  the  calamus 
scriptorius,  from  its  fancied  resemblance  in  shape  to  the 
nib  of  a  pen.  The  narrowed  upper  end  of  the  fourth  ven- 
tricle is  continued  forwards  under  the  cerebellum. 

Having  thus  made  out  so  much  of  the  arrangement  of 
the  brain  as  may  be  seen  by  mere  external  inspection,  we 
may  now  proceed  to  examine  its  internal  structure.  For 
this  purpose  the  most  instructive  method  is  to  cut  a  verti- 
cal, longitudinal  section  through  the  brain  from  front  to 
back,  passing  through  the  middle  line,  and  thus  dividing 
it  into  two  similar  and  symmetrical  halves.  When  the  cut 
surface  of  the  right  half  of  the  brain,  as  exposed  by  this 
section,  is  examined,  the  following  further  structural  details 
may  be  made  out,  and  are  shown  in  Fig.  166. 

The  corpus  cattosum  is  seen  cut  across  at  c.c,  'c.c,  c.c.   Above 


522 


ELEMENTARY   PHYSIOLOGY 


this,  and  extending  forwards  and  backwards,  is  the  flattened 
exposed  surface  of  the  right  cerebral  hemisphere,  which 
forms  one  side  of  the  median  fissure  between  the  hemi- 
spheres. The  upper  end  of  the  spinal  cord,  Sp.c,  passes 
into  the  bulb  B,  in  front  of  which  the  transverse  fibres  of 
the  pons  are  seen  in  section  at  P,  while  the  longitudinal 


Fig.  166.  —  View  of  the  Right  Half  of  a  Human  Brain  as  shown  b\  A 
Longitudinal  Section  in  the  Median  Line  through  the  Longitudinal 
Fissure.     (After  Sherrington.) 

Sp.c,  spinal  cord;  .5,  bulb;  P,  pons;  CR,  cms  cerebri;  M,  corpus  albicans;  CI. 
cerebellum;  c.c,  central  canal  of  spinal  cord  opening  into  4,  the  fourth  ventricle;  V.V. 
valve  of  Vieussens ;  QP,  QA ,  posterior  and  anterior  corpora  quadrigemina,  beneath 
which  is  the  aqueduct  of  Sylvius  leading  from  the  fourth  ventricle  into  3,  the  cavity 
of  the  third  ventricle;  P,  pineal  gland:  F,  fornix  or  roof  of  third  ventricle;  07] 
optic  thalamus;  H,  pituitary  body;  OP,  optic  nerve  cut  across  at  the  optic  decus- 
sation (see  Figs.  165  and  172) ;  SL,  a  part  of  the  septum  lucidum,  of  which  th< 
remainder  has  been  cut  away  to  reveal  JVC,  LV,  the  cavity  of  the  lateral  ventricle; 
this  communicates  with  the  third  ventricle  by  means  of  the  foramen  of  Monro 
whose  position  is  marked  by  a  small  x  at  the  front  end  of  the  third  ventricle;  c.c,  c.c, 
c.c,  corpus  callosum,  above  which  is  the  mesial  surface  of  the  right  cerebral  hemi 
sphere. 

fibres  of  the  bulb  run  forwards  above  the  pons  to  emerge 
in  front  as  one  of  the  (right)  crura  cerebri.  Anteriorly 
this  crus  disappears  out  of  the  section  since  it  diverges 
to  the  right  (see  Fig.  165)  from  the  median  line  of  the 
brain    to    enter    the    corresponding    cerebral    hemisphere. 


xii  THE  ANATOMY   OF  THE   BRAIN  523 

The  cerebellum,  Cb,  is  seen  in  section  overhanging  the 
bulb,  and  between  it  and  the  bulb  is  the  cavity,  shaded 
and  marked  with  a  4,  to  which  we  have  previously  alluded 
as  the  fourth  ventricle.  The  central  canal,  c.c,  of  the 
spinal  cord  is  shown  as  an  opening  into  the  hinder  end 
of  the  cavity  of  the  fourth  ventricle,  while  the  front  end 
of  the  cavity  is  prolonged  into  a  narrow  passage,  the  aque- 
duct of  Sylvius,  which  leads  into  a  much  larger  cavity, 
known  as  the  third  ventricle  and  marked  by  a  J.  Above 
this  aqueduct  are  four  largely  developed  masses  of  tissue, 
but  of  these  two  only  are  seen  in  the  section  at  QA,  QP, 
since  the  four  are  arranged  in  two  pairs,  one  pair  being 
placed  each  side  of  the  middle  line  of  the  brain ;  from 
their  number  (four)  these  structures  have  received  the 
name  of  corpora  quadrigemina.  In  front  of  the  corpora 
quadrigemina  is  a  small  structure,  seen  in  section,  the 
pineal  gland,  P.  The  posterior  corpus  quadrigeminum  is 
continuous  with  a  thin  layer  of  nervous  tissue  which  leads 
back  into  the  cerebellum  ;  this  forms  an  overhanging  roof 
to  the  front  end  of  the  fourth  ventricle,  and  is  known  as 
the  valve  of  Vieussens  (Fig.  166,  V.V).  The  floor  of  the 
third  ventricle  is  produced  forwards  and  downwards  into  a 
funnel-shaped  space,  to  the  tip  of  which  is  attached  a  body 
of  a  glandular  nature  known  as  the  pituitary  body  (Fig.  165, . 
P,  and  Fig.  166,  If).  The  roof  of  the  third  ventricle  is 
provided  by  a  layer  of  pia  mater,  called  the  velum  inter- 
positum,  and  not  shown  in  the  figure  ;  this  is  covered  by 
a  tract  of  fibres  seen  in  section  and  known  as  the  fornix 
(Fig.  1 66,  F)  ;  this  is  connected  posteriorly  with  the 
hinder  end  of  the  corpus  callosum,  and  in  front  it  curves 
downwards  and  backwards  into  the  lateral  wall  of  the  third 
ventricle.  The  vertical  space  between  the  fornix  and  the 
corpus  callosum  is  filled  in  by  a  thin  double  layer  of  ner- 


524  ELEMENTARY   PHYSIOLOGY  less. 

vous  tissue  ;  this  is  known  as  the  septum  lucidum.  It  lies 
in  the  plane  of  the  paper  on  which  the  figure  is  printed, 
but  only  a  small  portion  of  it  is  shown  at  SL.  The  remain- 
ing part  has  been  cut  away  in  order  to  reveal  a  feature  of 
which,  so  far,  no  mention  has  been  made,  viz.  the  darkly 
shaded  cavity  JVC,  LV,  lying  in  the  middle  of  the  cere- 
bral hemisphere,  and  known  as  the  lateral  ventricle.  The 
cavity  of  this  ventricle  communicates  with  that  of  the  third 


Fig.  167.  —  Diagram  to  show  the  Shape  of  the  Cavity  of  the  Left  Lateral 
Ventricle,  its  Connection  with  the  Third  Ventricle,  and  the  Connec- 
tion of  the  Latter  with  the  Fourth  Ventricle,  and  hence  with  the 
Central  Canal  of  the  Spinal  Cord. 

Drawn  from  a  cast  of  the  ventricles.     (After  Welcker.) 
c.c,  canal  of  spinal  cord;  4,  fourth  ventricle;   A. S.  aqueduct  of  Sylvius;  3,  third 

ventricle;   F.M,  foramen  of  Monro;  LP',  LV,  LV,  lateral  ventricle  with  its  anterior 

cornu  A.C,  posterior  cornu  P. C,  and  inferior  cornu  I.C. 

ventricle  by  a  small  opening  at  x,  the  foramen  of  Monro. 
Since  the  septum  lucidum  consists  of  two  layers,  there  is 
a  small  flattened  closed  space  between  these  layers  in  the 
middle  line  of  the  brain ;  this  is  spoken  of  as  the  fifth 
ventricle,  but  it  has  no  actual  connection  with  the  other 
ventricles.1  (See  Fig.  168,  5.)  Each  lateral  ventricle  is 
a  cavity  of  a  very  peculiar  shape,  one  branch  running  for- 
wards towards  the  front  end   of  the   hemisphere  and  one 

1  The  two  lateral  ventricles,  one  in  each  cerebral  hemisphere,  are  some- 
times reckoned  as  the  first  and  second  ventricles;  hence  the  space  between 
the  layers  oHhe  septum  lucidum  is  known  as  the  fifth  ventricle. 


xu  THE   ANATOMY   OF  THE   BRAIN  525 

backwards  towards  the  hinder  end,  and  from  the  latter  a  third 
branch  runs  downwards  and  once  more  forwards  (Fig.  167). 
These  correspond  respectively  to  the  chief  lobes  of  which 
each  hemisphere  is  made  up,  namely,  the  frontal  lobe,  the 
parietal  and  occipital  lobes,  and  the  temporal  lobe.  These 
lobes  are  marked  off  on  the  surface  of  the  hemispheres  by 
fissures,  of  which  the  most  conspicuous  are  the  fissure  of 
Sylvius,  and  the  fissure  of  Rolando.      (See   Fig.    173.) 

The  cerebellum  is  firmly  connected  to  the  rest  of  the 
brain  by  the  transverse  fibres  which  help  to  form  the  pons 
Varolii  (Fig.  165),  and  constitute  the  middle  peduncle  of 
each  half  of  the  cerebellum.  But  each  half  has  a  further 
attachment  by  means  of  two  other  bands  of  fibres.  Of  these 
one  coming  out  of  the  central  part  of  the  cerebellum  on 
each  side  runs  upwards  towards,  and  disappears  under,  the 
corpora  quadrigemina ;  this  forms  the  superior  peduncle. 
The  other  runs  downwards  towards  the  bulb  and  merges,  as 
the  inferior  peduncle,  into  that  part  of  the  bulb  which  is  a 
continuation  upwards  of  the  lateral  columns  of  white  matter 
of  the  spinal  cord. 

We  have  seen  that  the  spinal  cord  consists  essentially  of 
grey  matter  containing  nerve-cells,  external  to  which  is  a 
covering  of  white  matter  composed  of  nerve-fibres,  the 
arrangement  of  the  grey  and  white  matter  being  compara- 
tively simple,  and  the  nervous  tissue  surrounding  a  central 
canal.  Now  from  the  description  we  have  so  far  given  of 
the  brain,  it  is  evident  that  the  brain  may  also  be  regarded 
as  being  built  up  of  structures  which  are  placed  round  the 
sides  of  a  central  canal,  which  is  really  continuous  with  the 
canal  of  the  spinal  cord.  But,  unlike  the  latter,  the  canal 
of  the  brain,  consisting  of  the  ventricles  and  aqueduct,  is  not 
a  simple  straight  tube,  but  has  a  very  peculiar  shape  (Fig. 
167)0     Moreover,  although  the  brain  is  made  up  of  grej 


526  ELEMENTARY   PHYSIOLOGY  less. 

and  white  matters,  which  by  their  greater  or  less  develop- 
ment form  the  structures  of  varying  size  which  make  up  the 
brain  as  a  whole,  the  grey  and  white  matters  are  not  ar- 
ranged in  any  simple  way  as  they  are  in  the  spinal  cord. 
On  the  contrary,  although  in  the  brain  a  great  deal  of  the 
grey  matter  is  placed  externally  to  the  white,  the  latter  is 
interspersed  with  localised  deposits  of  grey  matter,  some 
large,  some  small,  which  give  to  the  whole  an  extraordinary 
complexity.  And  this  complexity  is  still  further  increased 
by  the  existence  of  strands  or  bundles  of  nerve-fibres,  which 
serve  to  interconnect  all  these  various  deposits  of  grey 
matter,  so  as  to  insure  the  possibility  of  co-ordinated  action 
between  all  the  individual  parts  of  which  the  brain  as  a 
whole  is  built  up.  It  would  be  neither  possible  nor  desira- 
ble to  attempt  to  deal  in  any  detail  in  this  book  with  the 
varied  arrangements  of  the  several  deposits  of  grey  matter 
in  the  brain,  and  with  their  connections  by  strands  of  white 
matter.  But  some  of  them  stand  out  so  conspicuously  as 
structures,  and  are  so  important  in  their  functions,  that  we 
must  of  necessity  take  them  into  consideration. 

The  Corpora  Quadrigemina.  —  These  have  already  been 
described  as  four  conspicuous  masses  of  tissue  lying  in  two 
pairs  above  the  aqueduct  of  Sylvius.  They  consist  of  depos- 
its of  grey  matter  in  the  otherwise  thin  wall  of  the  roof  of 
the  aqueduct.  Each  deposit  is  surrounded  by  white  matter, 
and  from  each  bands  of  fibres  run  obliquely  downwards  and 
forwards,  those  from  the  anterior  pair  of  the  corpora  making 
connection  with  structures  connected  with  the  optic  nerve 
(Fig.  165,  II),  while  those  from  the  posterior  pair  are  be- 
lieved to  make  similar  connections  with  the  nerves  con- 
cerned in  hearing  (Fig.  165,  VIII). 

The  Optic  Thalami.  —  The  longitudinal  fibres  of  the  bulb, 
passing  between  the  transverse  fibres  of  the  pons,  reappear, 


THE  ANATOMY   OF  THE   BRAIN 


527 


as  we  have  seen,  in  front  of  the  pons  as  the  crura  cerebri. 
These  diverge  from  the  middle  line  to  enter  the  cerebral 
hemispheres.  As  each  crus  passes  into  the  base  of  the  cor- 
responding hemisphere,  it  receives  on  its  upper  surface  a 
large  deposit  of  grey  matter  placed  somewhat  obliquely 
across  its  course  ;  this  mass  of  grey  matter   is   the   optic 


Fig.  168.  —  Diagram  of  a  Horizontal  Section  of  the  Brain  above  the  Floor 
of  the  Lateral  Ventricles.     (After  Hirschfeld  and  Leveille.) 

Sp.c,  spinal  cord;  B,  bulb;  Cb,  Cb,  cerebellum;  4,  fourth  ventricle;  QP,  QA, 
corpora  quadrigemina;  P,  pineal  gland;  3,  third  ventricle;  5,  fifth  ventricle;  cc, 
front  part  of  corpus  callosum;  LI',  LI',  LI',  lateral  ventricle:  OT,  optic  thalami; 
CS,  CS,  corpus  striatum;  C,  commissure  of  optic  thalami.  On  the  left  side  CS' 
marks  the  corpus  striatum,  into  which  an  incision  has  been  made  and  a  flap,_/",  turned 
back  to  show  its  internal  striated  appearance. 


thalamus.  Lying  thus  to  one  side  of  the  third  ventricle,  and 
under  the  lateral  ventricle,  it  is  easily  seen  how  each  optic 
thalamus  comes  to  form  a  projection  in  the  outer  side-wall 
of  the  third  ventricle,  and  on  the  floor  of  the  lateral  ventri- 
cle.     Thus  the  optic  thalamus  is  shown  at  OT  in  Fig.  166 


5 28  ELEMENTARY   PHYSIOLOGY  less 

as  part  of  the  wall  of  the  third  ventricle,  and  in  Fig.  168 
as  part  of  the  floor  of  the  lateral  ventricle,  the  latter  fig- 
ure representing  in  diagram  a  horizontal  section  through  the 
hemispheres  passing  above  the  floor  of  the  lateral  ventricles. 
The  inner  sides  of  the  optic  thalami  are  connected  by  a 
small  commissure  (Fig.  168,  C),  which  extends  across  the 
third  ventricle  ;  their  outer  sides  are  imbedded  in  the  sub- 
stance of  the  cerebral  hemispheres  with  which  they  are  con- 
nected by  nerve-fibres,  and  from  their  hinder  end  a  bundle 
of  fibres  sweeps  forwards  and  downwards  to  pass  into  the 
tract  of  the  optic  nerves. 

The  Corpora  Striata.  —  Each  corpus  striatum  may  be  re- 
garded as  a  mass  of  grey  matter  deposited  obliquely,  as  was 
each  opticf  thalamus,  on  the  course  of  the  crura  cerebri,  but 
lying  somewhat  in  front  of  the  optic  thalami.  Hence  the 
corpora  striata  are  seen  as  a  projection  on  the  floor  of  the 
lateral  ventricles  (Fig.  168,  CS,  OS'),  and  as  part  of 
the  side  wall  of  the  front  end  of  this  ventricle  (Fig.  166, 
JVC).  The  larger  part  of  each  corpus  striatum  is  imbedded 
in  the  neighbouring  substance  of  the  cerebral  hemisphere 
with  which  it  is  intimately  connected  by  nerve-fibres.  It  is 
also  similarly  connected  with  the  fibres  of  the  crus  on  which 
it  lies. 

The  Membranes  of  the  Brain.  —  The  brain  is  invested  by 
three  membranes  which  are  the  same  in  name,  and  similarly 
placed  and  related  to  each  other  as  those  which  we  have 
previously  described  as  covering  the  spinal  cord  (see  p. 
477).  Of  these  the  pia  mater  is  highly  vascular,  and  carries 
blood-vessels  down  into  the  nervous  matter,  especially  in  the 
sulci  or  grooves  to  which  the  convoluted  appearance  of  the 
surface  of  the  brain  is  due.  Moreover,  it  forms  a  roof  to 
the  hinder  part  of  the  cavity  of  the  fourth  ventricle,  and  a 
highly  developed  layer  of  the  pia  mater  is  tucked  in  under 


xii  THE   MINUTE   STRUCTURE  OF  THE   BRAIN         529 

the  hinder  end  of  the  cerebral  hemispheres  to  form  the  roof 
of  the  third  ventricle ;  this  is  known  as  the  velum  inter  - 
positum.  The  edges  of  this  velum  as  it  lies  beneath  the 
fornix  project  on  each  side  into  the  cavities  of  the  lateral 
ventricles  and  are  here  known  as  the  choroid  plexuses,  the 
whole  being  arranged  with  a  view  to  the  nutrition  of  the 
internal  parts  of  the  brain.  The  cavities  of  the  cerebral 
ventricles,  and  hence  of  the  central  canal  of  the  spinal  cord, 
are  placed  in  communication  with  the  subarachnoid  space 
by  a  small  opening  in  the  pia  mater  covering  the  hinder  end 
of  the  fourth  ventricle ;  this  opening  is  known  as  the 
foramen  of  Magendie. 

12.  The  Minute  Structure  of  the  Brain.  —  In  the  spinal 
bulb  the  arrangement  of  the  white  and  grey  matter  is  sub- 
stantially similar  to  that  which  obtains  in  the  spinal  cord, 
that  is  to  say,  the  white  matter,  composed  of  nerve-fibres, 
is  external  and  the  grey  internal ;  but  the  grey  matter, 
containing,  as  in  the  spinal  cord,  nerve-cells,  is  more 
abundant  than  in  the  spinal  cord,  and  the  arrangements  of 
white  and  grey  matter  become  much  more  intricate  and 
complex. 

Above  the  bulb  there  are  internal  deposits  of  grey  matter, 
containing  nerve-cells  at  various  places,  more  especially  in 
the  pons  Varolii,  the  crura  cerebri,  the  corpora  quadri- 
gemina,  optic  thalami,  and  corpora  striata.  And  there  is  a 
remarkably  shaped  deposit  of  grey  matter  in  the  interior  of 
the  cerebellum,  on  each  side.  But  what  especially  charac- 
terises the  brain  is  the  presence  of  grey  matter  of  a  special 
nature  on  the  surface  of  the  cerebral  hemispheres,  contain- 
ing peculiarly  shaped  nerve-cells,  and  known  as  the  cortex, 
and  similarly  a  special  grey  matter  forms  the  surface  of  the 
cerebellum.  This  superficial  grey  matter  covers  the  whole 
surface  of  both  these  organs,  dipping  down  into  the  fissures 

2M 


530  ELEMENTARY   PHYSIOLOGY  less. 

(sulci)  of  the  former,  and  following  the  peculiar  plaits  or 
folds  (convolutions)  into  which  the  latter  is  thrown. 

The  Cerebellum.  —  The  surface  of  the  cerebellum  presents 
a  corrugated  or  laminated  appearance.  When  a  section  is 
made  through  one  of  its  hemispheres  it  is  seen  that  the 
depressions  which  separate  the  laminse  give  off  secondary 
lateral  depressions  as  they  pass  towards  its  centre,  so  that 
the  surface  is  really  divided  up  into  a  very  large  number  of 
leaf-like  foldings  which  are  known  as  the  lamellae.  The 
central  part  of  the  cerebellum  consists  of  white  matter  which 
is  essentially  the  same  as  the  white  matter  of  the  spinal  cord, 
that  is  to  say,  it  is  made  up  chiefly  of  medullated  nerve- 
fibres.  Portions  of  this  white  matter  extend  outwards  into 
the  primary  foldings  and  secondary  lamellae  of  the  cerebellar 
surface,  and  are  covered  by  grey  matter,  the  arrangement 
thus  presenting  a  very  characteristic  arborescent  appearance 
when  seen  in  section.1 

When  a  section  of  the  external  grey  matter  is  cut  at  right 
angles  to  the  surface  of  a  lamella,  stained,  and  examined 
under  the  microscope,  it  is  found  to  consist  of  two  layers. 
The  innermost,  lying  next  to  the  central  white  matter,  is 
made  up  of  a  large  number  of  small,  closely  packed  cells 
supported  by  neuroglia  (see  p.  490)  and  is  known  as  the 
nuclear  layer  (Fig.  169,  N).  The  outer  layer,  immediately 
under  the  pia  mater,  shows  a  few  cells,  but  the  chief  appear- 
ance it  presents  is  that  of  a  granular  mass  made  up  of  closely 
set  dots.  These  dots  are  in  reality  the  cut  ends  of  fibres,  of 
which  some  belong  to  the  supporting  neuroglia,  but  of 
which  the  majority  are  nerve  fibrils.  From  its  punctated 
appearance  (x)  this  layer,  which  is  much  broader  than  the 
nuclear  layer,  is  known  as  the  molecular  layer  (M).  Be- 
tween these  two  layers  lies  a  row  of  nerve-cells  of  very  strik- 

l  This  is  somewhat  imperfectly  shown  in  Figs.  166  and  168. 


THE   CEREBELLUM 


53^ 


ing  and   characteristic   appearance,  known   as  the  cells  of 
Furkinje    (i).     These   are   pear-shaped,  with   a   large   and 


Fig.  169.  —  Diagram  to  illustrate  the  Structure  of  the  Superficial  Grey 
Matter  of  the  Cerebellum  as  seen  in  a  Transverse  Section  of  a 
Lamella. 

M,  molecular  layer;  N,  nuclear  layer;  W,  central  white  matter;  1,  cell  of  Pur- 
kinje;  2,  spider  cell ;  5,  cell  of  Golgi:  3,  basket-cell  with  one  of  its  baskets,  b\  4,  an- 
other kind  of  cell  in  the  molecular  layer;  t,  tendril  fibre;   m,  moss-fibre. 

In  the  case  of  each  cell  a  is  the  axon,  d  is  a  dendrite:  _r,  customary  punctated 
appearance  of  the  molecular  layer  when  seen  in  microscopic  sections. 


532  ELEMENTARY   PHYSIOLOGY  less. 

conspicuous  nucleus,  the  bulbous  inner  end  resting  on  the 
nuclear  layer,  while  the  outer  end  divides  into  a  large  num- 
ber of  processes  which  run  out  into  the  molecular  layer  as 
finer  and  finer  branches.  The  granular  appearance  of  the 
molecular  layer  is  in  part  due  to  the  close  juxtaposition  of 
the  cut  ends  of  these  branches  or  dendrites  from  the  cells  of 
Purkinje\  The  inner  end  of  each  cell  bears  a  single  process 
which  is  usually  cut  through  near  the  cell  but.  is  really  pro- 
longed down  into  the  central  white  matter  as  a  medullated 
nerve-fibre.  Such  are  the  details  which  can  be  made  out 
in  an  ordinarily  stained  section.  But  by  employing  special 
methods  of  staining  many  further  details  come  into  view, 
and  putting  all  these  together  we  are  justified  in  construct- 
ing the  preceding  diagrammatic  Figure  169  to  show  the 
nature  and  relationships  of  the  cells  of  the  cerebellar  cortex 
and  of  its  two  layers  to  the  fibres  of  the  central  white  matter. 

In  this  figure  the  cells  which  call  for  special  attention  are 
the  following :  The  cell  of  Purkinje  (1)  with  its  central 
axon  {a)  and  peripheral  dendrites  (d).  The  basket-cell 
(3)  with  its  axon  (a)  and  baskets  (b) ;  the  baskets  in 
reality  surround  the  bodies  of  cells  of  Purkinje,  which  for 
the  sake  of  clearness  are  not  shown  in  the  diagram.  The 
spider-cell  (2)  in  the  nuclear  layer  with  its  axon  (a)  running 
into  the  molecular  layer  and  dendrites  (d).  Also,  in  addi- 
tion to  the  fibre  derived-  from  the  inner  end  of  the  cell  of 
Purkinje^  it  is  important  to  notice  the  moss-fibre  (m)  whose 
outer  end  terminates  by  branching  in  the  nuclear  layer  and 
the  tendril-fibre  (7)  which  passes  further  outwards,  but  ends 
similarly  in  the  molecular  layer.  The  direction  in  which 
impulses  are  supposed  to  travel  along  these  fibres  is  indicated 
by  arrows. 

The  Cerebral  Cortex.  — The  structure  of  the  superficial  grey 
matter  of  the  cerebellum  is  practically  the  same  in  each  part 


THE   CEREBRAL   CORTEX 


533 


of  the   cerebellar  cortex.     In  the  cerebrum,  on  the  other 
hand,  the  details  of  structure  vary  not  inconsiderably,  ac- 


Fig.  170. — Diagrammatic  Figure  to  illustrate  the  Structure  of  a  Typical 
Section  of  the  Cerebral  Cortex. 

I,    Molecular  layer.       II,   Layer  of  pyramidal  cells.     Ill,    Layer  of  polymorphous 

cells. 
c  and  c'\  cells  of  the  molecular  layer;  /*•/">/'">  pyramidal  cells;  P,  cell  of  the 
polymorphous  layer:  My,  medullary  ray  of  nerve  fibrils  from  central  white  matter: 
x,  y,  z,  tangential  bundles  of  nerve  fibrils 


534  ELEMENTARY   PHYSIOLOGY  less. 

cording  to  the  region  of  the  cortex  from  which  a  section  is 
prepared.  Into  these  differences  we  cannot  enter,  but  must 
content  ourselves  with  a  somewhat  diagrammatic  description 
and  figure  in  illustration  of  the  general  structural  arrange- 
ment of  the  cells  and  fibres  of  the  cortex  as  a  whole. 

The  grey  matter  is  permeated  throughout  its  whole  thick- 
ness by  a  neuroglia  which  is  essentially  the  same  as  that  of 
the  rest  of  the  central  nervous  system.  This  forms  the 
supporting  tissue  in  which  the  nerve-cells  of  the  cortex  are 
imbedded,  and  through  which  the  fibrils  of  nerves  pass  to 
and  from  these  cells  from  and  to, the  central  white  matter. 
The  latter  is  composed,  as  in  the  cerebellum,  of  medullated 
nerve-fibres.  The  neuroglia  is  most  marked  in  the  outer- 
most parts  of  the  cortex,  immediately  below  the  pia  mater, 
and  since  in  a  section  its  wavy  fibres  are  mostly  seen  as 
sectional  dots,  this  layer  of  the  cortex  is  known  as  the 
molecular  layer  (Fig.  170,  I).  Internally  to  this  layer  the 
cortex  is  characterised  by  the  presence  of  nerve-cells  whose 
shape  is  pyramidal  with  the  apex  of  each  cell  pointed  to- 
wards the  surface  of  the  brain.  This  layer  may  therefore  be 
spoken  of  as  the  layer  of  pyramidal  cells  (Fig.  170,  II). 
These  cells  vary  in  size  in  the  several  parts  of  this  layer,  the 
largest  being  found  in  the  inner  portion,  the  smallest  next 
to  the  molecular  layer.  That  part  of  the  cortex  which  lies 
immediately  external  to  the  central  white  matter  is  character- 
ised by  the  presence  of  nerve-cells  of  a  somewhat  irregular 
form  ;  hence  this  layer  is  known  as  the  layer  of  polymorphous 
cells  (Fig.  170,  III). 

In  addition  to  the  nerve-cells  and  their  processes,  which 
characterise  the  several  layers  of  the  cortex,  nerve  fibrils 
pass  up  into  and  through  the  cortex  from  the  central  white 
matter.  Of  these  some  are  arranged  in  bundles  at  right 
angles  to  the  surface  of  the  cortex,  medullary  rays   (Fig. 


XII  THE   CRANIAL   NERVES  535 

170,  M/-),  while  others  lie  parallel  to  the  surface  as  tan- 
gential rays    (Fig.    170,  x,y,z). 

13.  The  Cranial  Nerves.  —  Nerves  are  given  off  from 
the  brain  in  pairs,  which  succeed  one  another  from  before 
backwards,  to  the  number  of  twelve  (Figs.  165  and  171). 
These  are  often  called  "cranial"  nerves,  to  distinguish 
them  from  the  spinal  nerves. 

The  first  pair,  counting  from  before  backwards,  are  the 
olfactory  nerves,  and  the  second  are  the  optic  nerves.  The 
functions  of  these  have  already  been  described.  The  ol- 
factory nerves  are  bundles  of  fibres  which  proceed  from  the 
under-surface  of  the  olfactory  lobes  of  the  cerebrum  (Fig. 
165,/)  and  traverse  the  cribriform  plate  to  be  distributed 
to  the  olfactory  mucous  membrane.  These  fibres  are  non- 
medullated  and,  with  the  olfactory  lobes,  are  in  a  certain 
sense  prolongations  of  the  cerebral  hemispheres. 

The  optic  "  nerve  "  is  also  properly  speaking  a  lobe  of 
the  brain,  being  an  outgrowth  in  the  embryo  from  the  walls 
of  the  third  ventricle.  It  retains  its  character  as  a  part  of 
the  central  nervous  system  in  so  far  as  its  fibres  have  no 
neurilemma. 

The  optic  nerve  from  each  eye  meets  its  fellow  nerve 
from  the  other  eye  at  the  base  of  the  brain  below  the 
third  ventricle.  Here  they  cross  each  other  in  what  is 
called  the  optic  chiasma  (covered  by  the  pituitary  body 
P  in  Fig.  165)  and  are  continued  on  backwards,  to  make 
connection  with  the  brain,  as  the  optic  tracts. 

These  are  connected,  as  already  stated,  with  the  hinder 
part  of  the  optic  thalami  and  with  the  anterior  pair  of 
the  corpora  quadrigemina.  At  the  chiasma  the  fibres  of  the 
optic  nerves  undergo  a  remarkable  partial  decussation.  The 
fibres  from  each  half  of  the  retina  nearest  to  the  nose  cross 
over  to  the  opposite  side  of  the  brain :  the  fibres  from  the 


536 


ELEMENTARY   PHYSIOLOGY 


other  half  of  each  retina  pass  into  the  brain  without  cross- 
ing. Hence  the  right  optic  tract  contains  the  fibres  from 
the  nasal  half  of  the  left  retina  and  from  the  other  or  tem- 
poral half  of  the  right  retina,  and,  similarly,  the  left  optic 
tract  is  made  up  of  the  fibres  of  the  temporal  half  of  the 
left  retina  and  the  nasal  half  of  the  right  retina.  This 
arrangement  is  essential  to  the  eye  as  a  sense  organ  with 
reference  to  what  we  have  previously  spoken  of  as  "  corre- 
sponding points  "  and  "  single  vision  with  two  eyes "  (see 
p.  472). 


Fig.  171.- 


■  A  Diagram  illustrating  the  Superficial  Origin  of  the  Cranial 
Nerves. 


H. ,  the  cerebral  hemispheres;  C.S.,  corpus  striatum;  77;.,  optic  thalamus;  P, 
pineal  body;  Pi,  pituitary  body;  C.Q.,  corpora  quadrigemina;  Cb,  cerebellum;  M, 
medulla  oblongata;  XII-I,  the  pairs  of  cerebral  nerves;  Sp  1,  Sp  2,  the  first  and 
second  pairs  of  spinal  nerves. 

The  third  pair  are  called  motor  oculi  (mover  of  the 
eye),  because  they  are  distributed  to  all  the  muscles  of 
the  eye  except  two. 

The  nerves  of  the  fourth  pair,  trochlear,  and  of  the 
sixth  pair,  abducens,  supply,  each,  one  of  the  muscles  of 
the  eye,  on  each   side ;   the  fourth   going  to  the  superior 


XII       -  THE  CRANIAL  NERVES  537 

oblique  muscle,  and  the  sixth  to  the  external  rectus. 
Thus  the  muscles  of  the  eye,  small  and  close  together  as 
they  are,  receive  their  nervous  stimulus  by  three  distinct 
nerves. 

Each  nerve  of  the  fifth  pair  is  very  large.  It  has  two 
roots,  a  motor  and  a  sensory,  and  further  resembles  a 
spinal  nerve  in  having  a  ganglion  on  its  sensory  root.  Its 
sensory  part  supplies  the  skin  of  the  face,  and  its  motor 
part  the  muscles  of  the  jaws,  and,  having  three  chief  divi- 
sions, it  is  often  called  trigeminal.  One  branch  containing 
sensory  fibres  supplies  the  fore-part  of  the  mucous  mem- 
brane of  the  tongue,  and,  from  its  supposed  share  in  medi- 
ating sensations  of  taste,  is  often  spoken  of  as  the  gustatory. 


Fig.  172. — Diagram  to  illustrate  the  Decussation  of  fibres  in  the  Optic 
Chiasma. 

R.  right  eye;  L,  left  eye;  R.op , ,  right  optic  tract;  L.op.,  left  optic  tract.  The 
decussation  is  shown  by  the  distribution  of  the  right  (shaded)  and  the  left  (unshaded) 
tract  to  the  retinas  of  the  two  eyes. 

The  seventh  pair  furnish  with  motor  nerves  the  muscles 
of  the  face  and  some  other  muscles,  and  are  called  facial. 

The  eighth  pair  are  the  auditory  nerves.  The  auditory 
is  divided  into  the  cochlear  and  vestibular  nerve.  (See 
later,  p.  542.) 

The  ninth  pair  in  order,  the  glossopharyngeal,  are  mixed 
nerves ;  each  being,  partly,  a  nerve  of  taste,  and  supplying 
the  hind-part  of  the  mucous  membrane  of  the  tongue,  and, 
partly,  a  motor  nerve  for  the  pharyngeal  muscles. 


53S  ELEMENTARY   PHYSIOLOGY  less. 

The  tenth  pair  are  the  two  pneumogastric  nerves,  often 
called  the  vagus.  These  very  important  nerves,  and  the 
next  pair,  are  the  only  cranial  nerves  which  are  distributed 
to  regions  of  the  body  remote  from  the  head.  The  pneu- 
mogastric has  the  widest  distribution  of  any  of  the  cranial 
or  spinal  nerves.  It  contains  both  afferent  and  efferent 
fibres,  and  supplies  the  larynx,  the  lungs,  the  liver,  the 
oesophagus,  stomach,  and  intestines,  and  branches  of  it  are 
connected  with  the  heart. 

The  eleventh  pair,  again,  called  spinal  accessory,  differ 
widely  from  all  the  rest,  in  arising  largely  from  the  sides  of 
the  spinal  cord,  between  the  anterior  and  posterior  roots  of 
the  dorsal  nerves.  They  run  up,  gathering  fibres  as  they 
go,  to  the  medulla  oblongata,  and  then  leave  the  skull  by 
the  same  aperture  as  the  pneumogastric  and  glossopharyn- 
geal. They  are  purely  motor  nerves,  supplying  certain 
muscles  of  the  neck. 

The  twelfth,  and  last,  pair,  the  hypoglossal,  are  the  motor 
nerves  which  supply  the  muscles  of  the  tongue. 

14.  The  Functions  of  the  Spinal  Bulb  or  Medulla 
Oblongata.  ■ —  The  bulb  plays  so  important  a  part  in  the 
economy  of  the  body  that  we  may  almost  enumerate  its 
functions  by  recalling  all  the  instances  in  which  we  have 
made  mention  of  its  activities  in  the  earlier  lessons  of  this 
book.  Thus,  we  have  seen  that  it  contains  a  centre  which 
gives  rise  to  the  contractions  of  the  respiratory  muscles  and 
keeps  the  respiratory  pump  at  work ;  hence  injuries  to  the 
bulb  may  arrest  the  respiratory  process  (p.  180).  Further, 
it  contains  centres  for  the  regulation  of  the  heart-beat  (p. 
101)  and  of  the  condition  of  the  blood-vessels  over  the 
whole  body  (p.  95).  But  beyond  these  the  bulb  also  con- 
tains centres  for  the  nervous  act  of  swallowing,  for  the  reflex 
secretion  of  saliva,  and  for  many  other  actions.     Thus,  we 


xii  THE   FUNCTIONS   OF  THE   SPINAL   BULB  539 

find  that  simple  puncture  of  one  side  of  the  floor  of  the 
fourth  ventricle  produces  for  a  while  an  increase  of  the 
quantity  of  sugar  in  the  blood  beyond  that  which  can  be 
utilised  by  the  organism.  The  sugar  passes  off  by  the  kid- 
neys, and  thus  this  slight  injury  to  the  medulla  produces  a 
temporary  disorder  closely  resembling  the  disease  called 
diabetes.  Hence  we  speak  of  a  diabetic  centre  in  the  bulb. 
Beyond  this  the  bulb  acts  as  a  great  conductor  of  impulses ; 
for  all  impulses  passing  up  and  down  between  the  higher 
parts  of  the  brain  and  the  spinal  cord  must  make  then  way 
through  the  bulb  from  or  to  the  spinal  nerves.  And  a  simi- 
lar statement  holds  good  for  impulses  along  the  cranial 
nerves,  with  the  exception  of  the  olfactory,  optic,  and  third 
and  fourth  nerves. 

The  impulses  which  pass  through  the  bulb  cross,  for  the 
most  part,  from  one  side  to  the  other  on  their  way  along  it. 
In  the  case  of  the  main  efferent  or  crossed  pyramidal  tract 
of  the  spinal  cord,  the  crossing  of  the  fibres  which  compose 
the  tract  takes  place  by  means  of  what  is  called  the  decus- 
sation of  the  pyramids  in  the  anterior  columns  of  the  bulb 
(Fig.  177).  This  point  is  indicated  in  Fig.  165  by  a  group 
of  small  converging  marks  on  the  surface  of  the  bulb  just 
above  the  cut  end  marked  M.  Similarly,  the  fibres  con- 
cerned in  the  transmission  of  afferent  impulses  largely  cross 
in  the  bulb  by  paths  which  are  varied,  but  of  which  one  is 
well  marked  as  the  sensory  decussation.  This  general 
decussation  of  efferent  and  afferent  fibres  leads  to  the  result 
that  disease  or  injury  of  one  side  of  the  brain  affects  the 
opposite  side  of  the  body.  Thus,  when,  as  not  unfrequently 
happens,  a  blood-vessel  gives  way  in  the  left  cerebral  hemi- 
sphere, leading  to  a  destruction  of  nervous  matter  there,  the 
result  is  that  the  right  arm,  and  right  leg,  and  right  side  of 
the  body  generally  are  paralysed,  that  is,  the  will  has  no 


54©  ELEMENTARY  PHYSIOLOGY  less. 

longer  any  power  to  move  the  muscles  of  that  side,  and 
impulses  started  in  the  skin  of  that  side  cannot  awaken  sen- 
sations in  the  brain. 

But  there  is  also  a  decussation  of  impulses  in  the  case  of 
the  nerves  arising  from  the  medulla  above  the  decussation 
of  the  pyramids.  Thus,  in  the  case  quoted  above  of  a  blood- 
vessel bursting  in  the  left  cerebral  hemisphere,  the  right  side 
of  the  man's  face  is  paralysed  as  well  as  the  right  side  of  his 
body,  that  is  to  say,  impulses  cannot  pass  to  and  from  his 
brain  and  the  right  facial  and  fifth  nerves.  The  impulses 
along  these  nerves  also  cross  over,  decussate,  and  reach  the 
left  side  of  the  brain. 

It  sometimes  happens,  however,  that  disease  or  injury 
may  affect  the  medulla  oblongata  itself,  on  one  side  only 
{e.g.  the  left),  above  the  decussation  of  the  pyramids,  in 
such  a  way  that  the  fifth  and  facial  nerves  are  affected  in 
their  course  before  they  decussate,  that  is  to  say,  on  the 
same  side  as  the  injury.  The  man  then,  while  still  paralysed 
on  the  right  side  of  his  body,  is  paralysed  on  the  left  side  of 
his  face. 

15.  The  Functions  of  the  Cerebellum.  — When  speaking 
of  reflex  actions  we  pointed  out  (p.  510)  that  the  compli- 
cated movements  of  walking  when  once  started  by  the  will 
are  essentially  reflex  in  their  continued  production.  More- 
over, we  also  drew  attention  to  the  fact  that  the  co-ordination 
of  the  efferent  impulses  which,  although  distributed  to  many 
different  muscles,  give  rise  by  their  united  action  to  the 
orderly  movements  of  walking,  is  dependent  upon  afferent 
impulses  from  various  parts  of  the  body.  Thus,  walking  be- 
comes unsteady  or  even  impossible  in  the  absence  of  the 
normal  sensory  impulses  from  the  skin,  or  of  visual  impulses 
from  the  eyes ;  and  to  these  we  might  have  added  afferent 
impulses  from  the  sensory  nerves  of  the  muscles  themselves. 


xii  THE   FUNCTIONS   OF  THE  CEREBELLUM  541 

When  we  take  cases  of  movements  which  are  less  obviously 
reflex,  that  is,  more  strictly  voluntary,  than  are  those  of 
walking,  we  find  that  here  again  their  orderly  or  co-ordinated 
production  depends  largely  on  tactile  and  visual  impulses. 
Now  experiment  and  observation  in  cases  of  disease  have 
shown  quite  conclusively  that  the  one  great  function  of  the 
cerebellum  is  to  play  a  most  important  part  in  the  co-ordina- 
tion  of  the  actions,  nervous  and  muscular,  by  which  the 
movements  of  the  body  are  carried  on. 

After  the  cerebellum  has  been  completely  removed,  an 
animal  does  not  differ  in  any  essential  respect  from  its 
normal  condition  as  regards  its  intelligence  or  its  special 
senses,  such  as  sight  or  hearing.  But  with  regard  to  its 
movements  a  great  difference  is  observed ;  all  movements 
are  now  clumsily  executed  —  there  is  a  want  of  orderliness 
or  co-ordination.  The  above  statement  sums  up  our  knowl- 
edge of  the  function  of  the  cerebellum. 

We  do  not  know  how  the  cerebellum  works  in  thus  keep- 
ing an  orderly  grip  over  the  mechanisms  of  movement ;  but 
we  see  how  easily  it  may  do  so  when  we  consider  its  con- 
nections with  the  spinal  cord  and  with  the  rest  of  the  brain. 
We  saw  (p.  512)  that  two  large  tracts  of  afferent  fibres  from 
the  spinal  cord  pass  into  it.  Moreover,  it  is  connected  with 
that  part  of  the  bulb  in  which  the  postero-median  tract  ends. 
Thus  it  may  be  a  recipient  of  a  vast  number  of  afferent  sen- 
sory impulses,  which  are  so  essential  for  co-ordinated  move- 
ment. But  each  half  of  the  cerebellum  is  further  connected 
with  the  cortex  of  the  cerebral  hemisphere  of  the  opposite 
side.  And  we  shall  see  that  it  is  exactly  in  the  cortex  of 
the  cerebral  hemispheres  that  impulses  chiefly  arise  for  the 
initiation  of  muscular  movements. 

When  describing  the  arrangements  of  the  internal  ear,  it 
was  stated  that  the  semicircular  canals,  the  utricle,  and  the 


542  ELEMENTARY    PHYSIOLOGY  less. 

saccule  enable  the  body  to  maintain  its  equilibrium  (p.  421). 
Now  the  auditory  nerve  consists  of  two  quite  distinct  parts, 
the  cochlear  nerve,  which  is  distributed  to  the  cochlea,  and 
the  vestibular  nerve,  which  is  distributed  to  the  above-men- 
tioned parts  of  the  ear.  These  two  nerves  originate  in 
groups  of  cells  lying  in  the  spinal  bulb,  and  the  group  of 
cells  which  gives  rise  to  the  vestibular  nerve  is  directly  con- 
nected by  a  strand  of  fibres  with  the  cerebellum.  Thus 
there  is  a  path  by  which  afferent  (sensory)  impulses  from 
the  vestibular  organs  and  the  canals  may  reach  the  cere- 
bellum directly  and  there  be  turned  to  account  in  the  co- 
ordination of  movements.  Bearing  this  in  mind,  it  is  not 
surprising  to  find  that  these  organs  play  a  very  important 
part  in  the  guidance  of  co-ordinated  movement. 

16.  The  Functions  of  the  Cerebral  Hemispheres.  — 
The  Hemispheres  the  Seat  of  Intelligence  and  Will.  — The  func- 
tions of  most  of  the  parts  of  the  brain  which  lie  in  front  of 
the  spinal  bulb  are,  at  present,  very  ill  understood  ;  but  it  is 
certain  that  extensive  injury,  or  removal,  of  the  cerebral 
hemispheres  puts  an  end  to  intelligence  and  voluntary 
movement,  and  leaves  the  animal  in  the  condition  of  a 
machine,  working  by  the  reflex  action  of  the  remainder 
of  the  cerebro-spinal  axis. 

We  have  seen  that  in  the  frog  the  movements  of  the  body 
which  the  spinal  cord  alone,  in  the  absence  of  the  whole  of 
the  brain,  including  the  bulb,  is  capable  of  executing,  are 
of  themselves  strikingly  complex  and  varied.  But  none  of 
these  movements  arise  from  changes  originating  within  the 
organism ;  they  are  not  what  are  called  voluntary  or  sponta- 
neous movements ;  they  never  occur  unless  the  animal  be 
stimulated  from  without.  Removal  of  the  cerebral  hemi- 
spheres is  alone  sufficient  to  deprive  the  frog  of  all  sponta- 
neous or  voluntary  movements ;  but  the  presence  of  the 


xii      FUNCTIONS  OF  THE  CEREBRAL   HEMISPHERES     543 

bulb  and  other  parts  of  the  brain  (such  as  the  corpora 
quadrigemina,  or  what  corresponds  to  them  in  the  frog,  and 
the  cerebellum)  renders  the  animal  master  of  movements 
of  a  far  higher  nature  than  when  the  spinal  cord  only  is  left. 
In  the  latter  case  the  animal  does  not  breathe  when  left  to 
itself,  lies  flat  on  the  table  with  its  fore-limbs  beneath  it  in 
an  unnatural  position  ;  when  irritated  kicks  out  its  legs,  and 
may  be  thrown  into  actual  convulsions,  but  never  jumps 
from  place  to  place  ;  when  thrown  into  a  basin  of  water 
falls  to  the  bottom  like  a  lump  of  lead,  and  when  placed 
on  its  back  will  remain  so,  without  making  any  effort  to  turn 
over.  In  the  former  case  the  animal  sits  on  the  table,  rest- 
ing on  its  front  limbs,  in  the  position  natural  to  a  frog ; 
breathes  quite  naturally ;  when  pricked  behind  jumps  away, 
often  getting  over  a  considerable  distance ;  when  thrown 
into  water  begins  at  once  to  swim,  and  continues  swim- 
ming until  it  finds  some  object  on  which  it  can  rest ;  and 
when  placed  on  its  back  immediately  turns  over  and  re- 
sumes its  natural  position.  Not  only  so,  but  the  following 
very  striking  experiment  may  be  performed  with  it :  Placed 
on  a  small  board  it  remains  perfectly  motionless  so  long  as 
the  board  is  horizontal ;  if,  however,  the  board  be  gradually 
tilted  up  so  as  to  raise  the  animal's  head,  directly  the  board 
becomes  inclined  at  such  an  angle  as  to  throw  the  frog's 
centre  of  gravity  too  much  backwards,  the  creature  begins 
slowly  to  creep  up  the  board,  and,  if  the  board  continues  to 
be  inclined,  will  at  last  reach  the  edge,  upon  which,  when 
the  board  becomes  vertical,  he  will  seat  himself  with  appar- 
ent great  content.  Nevertheless,  though  his  movements 
when  they  do  occur  are  extremely  well  combined  and  appar- 
ently identical  with  those  of  a  frog  possessing  the  whole  of 
his  brain,  he  never  moves  spontaneously,  and  never  stirs 
unless  irritated. 


544  ELEMENTARY   PHYSIOLOGY  less. 

Thus,  the  parts  of  the  brain  below  the  cerebral  hemi- 
spheres constitute  a  complex  nervous  machinery  for  carry- 
ing out  intricate  and  orderly  movements,  in  which  afferent 
impulses  play  an  important  part,  though  they  do  not  give 
rise  to  clear  or  permanent  affections  of  consciousness. 

There  can  be  no  doubt  that  the  cerebral  hemispheres  are 
the  seat  of  powers  essential  to  the  production  of  those  phe- 
nomena which  we  term  intelligence  and  will ;  and  there  is 
experimental  and  other  evidence  which  indicates  a  connec- 
tion between  particular  parts  of  the  surface  of  the  cerebral 
hemispheres  and  particular  acts.  Thus,  as  we  shall  see  more 
fully  later,  irritation  of  particular  spots  in  the  anterior  part 
of  a  dog's  brain  will  give  rise  to  particular  movements  of 
this  or  that  limb,  or  of  this  or  that  group  of  muscles ;  and 
the  destruction  of  a  certain  part  of  the  posterior  lobes  of  the 
cerebral  hemispheres  causes  blindness.  But  the  exact  way 
in  which  these  effects  are  brought  about  is  not  yet  thor- 
oughly understood ;  and  although  it  seems  to  be  proved 
beyond  doubt  that  the  central  end-organ  of  vision  (p.  448) 
consists  of  certain  nerve-cells  lying  in  a  particular  part  of 
the  posterior  surface  of  the  cerebral  hemisphere,  and  that 
the  central  end-organ  of  hearing  '(p.  414)  consists  of  other 
nerve-cells  lying  elsewhere  on  the  cerebral  surface,  we  are 
still  completely  in  the  dark  as  to  what  goes  on  in  the  cere- 
bral hemispheres  when  we  think  and  when  we  will. 

There  is  no  doubt  that  a  molecular  change  in  some  part 
of  the  cerebral  substance  is  an  indispensable  accompani- 
ment of  every  phenomenon  of  consciousness.  And  it  is 
possible  that  the  progress  of  investigation  may  enable  us 
to  map  out  the  brain  according  to  the  psychical  relations 
of  its  different  parts.  But  supposing  we  get  so  far  as  to 
be  able  to  prove  that  the  irritation  of  a  particular  frag- 
ment of  cerebral  substance  gives  rise  to  a  particular  state 


xn      FUNCTIONS  OF  THE  CEREBRAL  HEMISPHERES     545 

of  consciousness,  the  reason  of  the  connection  between 
the  molecular  disturbance  and  the  psychical  phenomenon 
appears  to  be  out  of  the  reach,  not  only  of  our  means  of 
investigation,  but  even  of  our  powers  of  conception. 

Reflex  Actions  of  the  Brain.  Even  while  the  cerebral 
hemispheres  are  entire,  and  in  full  possession  of  their 
powers,  the  brain  gives  rise  to  actions  which  are  as  com- 
pletely reflex  as  those  of  the  spinal  cord. 

When  the  eyelids  wink  at  a  flash  of  light  or  a  threatened 
blow,  a  reflex  action  takes  place,  in  which  the  afferent 
nerves  are  the  optic,  the  efferent  the  facial.  When  a  bad 
smell  causes  a  grimace,  there  is  a  reflex  action  through 
the  same  motor  nerve,  while  the  olfactory  nerves  consti- 
tute the  afferent  channels.  In  these  cases,  therefore,  reflex 
action  must  be  effected  through  the  brain,  all  the  nerves 
involved  being  cerebral. 

When  the  whole  body  starts  at  a  loud  noise,  the  affe- 
rent auditory  nerve  gives  rise  to  an  impulse  which  passes 
to  the  medulla  oblongata,  and  thence  affects  the  great 
majority  of  the  motor  nerves  of  the  body. 

It  may  be  said  that  these  are  mere  mechanical  actions, 
and  have  nothing  to  do  with  the  operations  which  we 
associate  with  intelligence.  But  let  us  consider  what  takes 
place  in  such  an  act  as  reading  aloud.  In  this  case,  the 
whole  attention  of  the  mind  is,  or  ought  to  be,  bent  upon 
the  subject-matter  of  the  book,  while  a  multitude  of  most 
delicate  muscular  actions  are  going  on  of  which  the  reader 
is  not  in  the  slightest  degree  aware.  Thus,  the  book  is 
held  in  the  hand,  at  the  right  distance  from  the  eyes ;  the 
eyes  are  moved  from  side  to  side,  over  the  lines  and  up 
and  down  the  pages.  Further,  the  most  delicately  adjusted 
and  rapid  movements  of  the  muscles  of  the  lips,  tongue,  and 
throat,  of  the  laryngeal  and  respiratory  muscles,  are  involved 

2N 


546  ELEMENTARY   PHYSIOLOGY  less. 

in  the  production  of  speech.  Perhaps  the  reader  is  stand- 
ing up  and  accompanying  the  lecture  with  appropriate  gest- 
ures. And  yet  every  one  of  these  muscular  acts  may  be 
performed  with  utter  unconsciousness,  on  his  part,  of  any- 
thing but  the  sense  of  the  words  in  the  book.  In  other 
words,  they  are  reflex  acts. 

Similar  remarks  apply  to  the  act  of  "  playing  at  sight " 
a  difficult  piece  of  music.  The  reflex  actions  proper  to 
the  spinal  cord  itself  are  natural,  and  are  involved  in  the 
structure  of  the  cord  and  the  properties  of  its  constituents. 
By  the  help  of  the  brain  we  may  acquire  an  infinity  of  arti- 
ficial reflex  actions ;  that  is  to  say,  an  action  may  require 
all  our  attention  and  all  our  volition  for  its  first,  or  sec- 
ond, or  third  performance,  but  by  frequent  repetition  it 
becomes,  in  a  manner,  part  of  our  organisation,  and  is 
performed  without  volition,  or  even  consciousness. 

As  every  one  knows,  it  takes  a  soldier  a  long  time  to 
learn  his  drill  —  for  instance,  to  put  himself  into  the  atti- 
tude of  "  attention  "  at  the  instant  the  word  of  command 
is  heard.  But,  after  a  time,  the  sound  of  the  word  gives 
rise  to  the  act,  whether  the  soldier  be  thinking  of  it  or 
not.  There  is  a  story,  which  is  credible  enough,  though 
it  may  not  be  true,  of  a  practical  joker,  who,  seeing  a  dis- 
charged veteran  carrying  home  his  dinner,  suddenly  called 
out  "  Attention  !  "  whereupon  the  man  instantly  brought  his 
hands  down,  and  lost  his  mutton  and  potatoes  in  the  gut- 
ter. The  drill  had  been  thorough,  and  its  effects  had 
become  embodied  in  the  man's  nervous  structure. 

The  possibility  of  all  education  (of  which  military  drill 
is  only  one  particular  form)  is  based  upon  the  existence  of 
this  power  which  the  nervous  system  possesses,  of  organ- 
ising conscious  actions  into  more  or  less  unconscious,  or 
reflex,  operations.     It  may  be  laid  down  as  a  rule,  which 


xii      FUNCTIONS   OF  THE   CEREBRAL    HEMISPHERES     547 

is  called  the  Law  of  Association,  that  if  any  two  mental 
states  be  called  up  together,  or  in  succession,  with  due 
frequency  and  vividness,  the  subsequent  production  of  the 
one  of  them  will  suffice  to  call  up  the  other,  and  that 
whether  we  desire  it  or  not. 

The  object  of  intellectual  education  is  to  create  such 
indissoluble  associations  of  our  ideas  of  things,  in  the  order 
and  relation  in  which  they  occur  in  nature ;  that  of  a  moral 
education  is  to  unite  as  fixedly  the  ideas  of  evil  deeds  with 
those  of  pain  and  degradation,  and  of  good  actions  with 
those  of  pleasure  and  nobleness. 

Localisation  of  Function  in  the  Cortex  of  the  Cerebral  Hemi- 
spheres.—We  have  already  alluded  (p.  544)  to  the  fact  that 
there  is  a  connection  between  particular  parts  of  the  sur- 
face of  the  cerebral  hemispheres  and  particular  acts  or 
special  sensations.  The  possibility  thus  indicated  is  of 
extraordinary  importance  and  must  now  be  dealt  with  in 
some  detail. 

The  cerebral  hemispheres  are  separated  along  the  mid- 
dle line  of  the  brain  by  a  narrow  deep  fissure,  across  which 
the  corpus  callosum  passes  as  a  bridge  from  one  hemisphere 
to  the  other  (see  Figs.  165  and  166).  The  surface  of  each 
hemisphere  is  folded  into  a  large  number  of  convolutions 
or  gyri  separated  from  each  other  by  sinuous  depressions 
or  sulci  (see  Fig.  164,  C,  C).  Some  of  these  depressions 
are  deeper  and  more  marked  than  others,  and  are  spoken 
of  as  fissures.  Of  these  the  most  conspicuous  are  known  as 
the  fissure  of  Sylvius,  the  fissure  of  Rolando,  the  parieto- 
occipital fissure,  and  the  calcarine  fissure.  The  position 
of  these  is  shown  in  the  accompanying  diagrams  (Figs.  173 
and  174).  These  fissures  may  be  taken  as  roughly  dividing 
the  surface  of  the  brain  more  or  less  distinctly  into  several 
lobes,  frontal,  parietal,  occipital,  and  temporal. 


548 


ELEMENTARY    PHYSIOLOGY 


When  the  surface  of  the  hemisphere  is  stimulated  elec- 
trically close  to  the  fissure  of  Rolando  and  on  either  side 
of  this  fissure,  very  definite  movements  take  place  in  the 
limbs  of  the  opposite  side  of  the  body.     If  care  is  taken  to 


Parieto-occipita.1 
Fissure 


Fissure    of 
Sylvius 


Fig.  173.  —  Diagram  of  Outer  Surface  of  the  Right  Cerebral  Hemisphere 


Fissure  of   Rolando 


oil 


^"CORU    LOBE 


Fig.  174.  —  Diagram  of  the  Inner   (Mesial)    Surface  of  the  Right  Hemi- 
sphere TO  SHOW  THE  PaRIETO-OCCIPITAL  AND  CaLCARINE  FlSSURES. 

The  corpus  c  alio  sum  is  seen  shaded  in  section. 


localise  the  stimulation  as  far  as  possible  within  the  limits 
of  a  small  area  of  the  cortex,  the  resulting  movements  are 
found  to  be  limited  to  a  correspondingly  small  group  of 
muscles  of  the  limb  affected.  Again,  if  that  piece  of  cor- 
tex whose  stimulation  gives  rise  to  movements  be  cut  out 


xn     FUNCTIONS   OF  THE   CEREBRAL   HEMISPHERES     549 

or  extirpated,  the  animal  so  operated  on  is  found  to  have 
lost  the  power  of  executing  this  particular  set  of  move- 
ments. The  outcome  of  such  experiments  makes  it  clear 
that  the  cerebral  cortex  along  the  course  of  the  fissure  of 
Rolando  is  concerned  in  the  development  of  muscular 
movements  ;  hence  the  name  of  "  motor  areas  "  was  given 
to  these  parts  of  the  cortex  (Figs.  175,  176).  Our  knowl- 
edge of  the  existence  and  position  of  these  areas  as  de- 
rived from  experiments  on  animals  is,  moreover,  completely 
confirmed  by  the  observation  of  the  results  of  Nature's  own 
experiments  on  man ;  as,  for  instance,  by  an  examination 
after  death  of  the  brains  of  patients  who  during  life  had, 
as  the  result  of  cerebral  disease,  exhibited  symptoms  simi- 
lar to  those  obtainable  by  stimulation  or  extirpation  of 
cortical  areas  in  animals. 

By  proceeding  in  a  similar  way  it  has  been  found  further 
that  certain  portions  of  the  cortex  are  peculiarly  connected 
with  the  development  of  sensations,  so  that  we  come  to 
speak  also  of  "sensory  areas"  (Figs.  175,  176).  In  this 
case  observations  on  man  are  specially  instructive,  since  the 
patient  can  give  an  account  of  his  sensations,  whereas 
another  animal  cannot. 

One  of  the  earliest  known  and  most  interesting  cases  of 
localisation  of  function  in  the  cerebral  cortex  is  that  of  the 
centre  for  speech.  Some  long  time  before  experiment  re- 
vealed the  existence  and  position  of  the  centres  to  which 
we  have  so  far  referred,  it  was  noticed  by  a  French  physi- 
cian named  Broca  that  patients  who  had  exhibited  a  curious 
inability  to  pronounce  definite  words  or  syllables  during  life 
were  found  after  death  to  have  suffered  from  disease  or 
injury  of  the  inferior  frontal  convolution  of  the  left  side  of 
the  brain  immediately  above  the  Sylvian  fissure ;  hence, 
this  part  of  the  cortex  is  known  as  Broca's  convolution  (see 


55° 


ELEMENTARY   PHYSIOLOGY 


Fig-    x75)-      The    disorder   is,  from    its   nature,  known  as 
aphasia  (a,  without,  <£acns,  speech)  and  may  take  one  of 


F/&SI/REOF  ftOLAf/DO 


Oc.L 


Te.L 


Fig.  175.  —  Diagram  of  Outer  Surface  of  Right  Cerebral  Hemisphere  to 
show  the  Position  of  Cortical  Areas. 

The  areas  for  the  leg,  arm,  and  face  are  marked  by  vertical  lines,  horizontal  lines, 
and  dots,  respectively.     The  area  for  speech  lies  really  on  the  left  hemisphere. 
Fr.L,  frontal  lobe;   Oc.L,  occipital  lobe;    Te.L,  temporal  lobe;  Sy.F,  fissure  of 
Sylvius. 


Fissure  of  Rolando 


Fr.L 


Par  Oc  F 


OcL 


TeL 


Fig.  176.  —  Diagram  of    Inner   (Mesial)    Surface  of   the   Right  Cerebral 
Hemisphere  to  show  the  Position  of  Cortical  Areas. 

The  corpus  callosum  is  seen  shaded  in  section. 
Fr.L,  frontal  lobe;  Oc  L,  occipital  lobe;    Te  L,  temporal  lobe;    Par-Oc  F.   parieto- 
occipital fissure. 


several  forms  ranging  from  complete  inability  to  speak  at  all 
to  an  inability  to  utter  certain  words,  and  hence  to  speak 


xii         THE    PATHS   OF   CONDUCTION   OF   IMPULSES        551 

coherently.  This  centre  for  speech  is,  curiously,  and  unlike 
most  of  the  other  centres,  unilateral,  being  situated  on  the 
left  side  of  the  brain  in  ordinary  right-handed  persons  and 
in  the  corresponding  part  of  the  right  side  of  the  brain  in 
those  who  are  left-handed. 

17.   The  Paths  of  Conduction  of  Impulses  in  the  Brain. 
—  Corresponding  to  the  greater  complexity  of  the  brain  in 


Int.  Cap 


Cr.p.  Bu]b 

Sp.C 

Fig.  177. — Diagram  of  the  Course  of  the  Crossed  Pyramidal  Tract  from 
the  (Motor)  Cerebral  Cortex  to  the  Spinal  Cord. 

Cb.H.,  cerebral  hemisphere;  C.C.,  corpus  callosum;  O.T.,  optic  thalamus;  C.S., 
corpus  striatum;  Int.  Cap,  internal  capsule;  Cb,  cerebellum;  D.P.,  decussation  of 
the  pyramids;  Cr.p. ,  Cr.p.,  crossed  pyramidal  tracts  (see  Fig.  162);  Sp.C,  spinal  cord. 


general,  as  compared  with  the  spinal  cord,  the  paths  of  con- 
duction in  the  former  are  much  more  numerous  and  intri- 
cate than  in  the  latter ;  and  one  of  the  chief  problems  of 
the  neurology  of  the  present  day  is  to  trace  out  these  paths. 
We  have  already  referred  incidentally  to  some  of  these. 

Many  of  the  sensory  fibres  of  the  spinal  cord,  after  cross- 
ing from  one  side  to  the  other  in  the  sensory  decussation 
(p.  539)  in  the  spinal  bulb,  can  be  traced  upwards  into  the 


552 


ELEMENTARY   PHYSIOLOGY 


UAiku** 


MukUuu^ 


"hvdulU  t-Uou^utt 


n 


Fig.  178. — Diagram  showing  the  Probable  Relation 
Principal  Cells  and  Fibres  of  the  Cerebro-spinal 
One  Another  (Schafer).     (From  Quain's  Anatomy.') 

s, :;  sensory  surface,  such  as  the  skin;  7,  8,  afferent  fibre  belonging  ft^^Lbody  b 
which  lies  in  ganglion  of  posterior  root  of  spinal  nerve;  8,  ascends  spinal  con 
tero-median  tract  (Fig.  161,  p.m.);  9,  10,  11,  12,  13,  branches  of  8  in  spinal"  6 
terminating  about  cell-bodies  in  posterior  horn  (9,  12,),  anterior  horn  (10),  Clarke' 
column  (11),  and  bulb  (13);  14,14,  relations  of  certain  cells  of  posterior  horn  to  those 
of  anterior  horn;  17,  neuron  of  bulb  sending  fibre  to  one  of  the  small  cells  in  cortex 
of  cerebrum;  18,  relation  of  small  cell  to  large  pyramidal  cell  1;  2,  axis-cylinder 
process  of  1  giving  off  branch  {call.)  to  corpus  callosum  and  thence  to  cortex  of 
opposite  hemisphere,  branch  (sir.)  to  corpus  striatum,  then  proceeding  downwards 
through  bulb  and  cord  in  pyramidal  tract,  and  finally  dividing  into  branches  3,  3,  3,  4, 
which  end  about  cells  of  anterior  horn;  5,  5,  5,  5,  efferent  fibres  from  cell-bodies  in 
anterior  horn  going  to  peripheral  organs,  e.g.  to  muscle,  in;  15,  ascending,  and  16, 
descending  fibres  in  relation  with  cells  of  Purkinjd  in  cerebellum.  The  simplest 
reflex  action  would  involve  at  least  7,  8,  10,  and  5. 


xii         THE    PATHS    OF   CONDUCTION   OF   IMPULSES        553 

cerebral  hemispheres  and  ultimately  to  the  sensory  areas  of 
the  cerebral  cortex.  Conversely,  many  fibres  from  the 
motor  areas  can  be  followed  as  a  very  definite  tract,  the 
pyramidal   tract,  downwards  through  the  crura  cerebri  to 


Fig.  X7Q. — Diagram  showing  Nervous  Inter-relations  of  Sense-organs  and 
Motor  organs.    (From  Landois  &  Sterling's  Text-book  0/ Human  Physiology). 

s,  s',  s",  a,  paths  of  sensory  impulses  going  to  brain;  m,  m',  paths  of  motor  im- 
pulses from  brain  to  muscles  of  lips  and  hand:  within  brain  are  centres  of  sight  (V), 
hearing  (A),  for  muscles  of  hand  (W),  and  for  speech  (E).  Arrows  indicate  the 
direction  of  the  nervous  impulses. 


the  decussation  of  the  pyramids  in  the  bulb,  and  thence  to 
the  descending  columns  of  the  cord.     These  sensory  and 


554  ELEMENTARY   PHYSIOLOGY  less,  xn 

motor  fibres  together  converge  in  a  fan-like  manner  from 
their  respective  cortical  terminations  into  a  large  and  very 
pronounced  bundle  between  the  optic  thalamus  and  corpus 
striatum  on  each  side,  which  is  known  as  the  internal  cap- 
sule (Fig.  177). 

We  have  already  referred  to  the  corpus  callosum  and  the 
pons  Varolii  as  composed  of  commissural  fibres  connecting 
the  two  halves  of  the  cerebrum  and  cerebellum  respectively. 
These  are  the  largest  of  several  commissures,  besides  which 
large  numbers  of  commissural  fibres  are  not  collected  into 
definite  bundles. 

One  of  the  most  interesting  of  all  the  various  pathways  is 
that  of  the  so-called  association  fibres,  which  run  between 
different  parts  of  the  same  hemisphere  in  both  the  cerebrum 
and  cerebellum.  These  constitute  definite  tracts,  by  which 
the  various  sensory  and  motor  areas  are  connected,  and  the 
harmonious  action  of  the  parts  is  assured. 

Figure  178  shows  very  diagrammatically  the  cellular  rela- 
tionships of  some  of  the  parts  of  the  cerebro-spinal  system  to 
one  another.  Figure  179  shows,  also  very  diagrammatically, 
how  the  centres  of  sight  and  of  hearing  may  be  associated 
with  each  other,  and  with  the  motor  areas  concerned  in 
speech  and  writing. 


APPENDIX 

ANATOMICAL   AND  PHYSIOLOGICAL    CONSTANTS 

The  weight  of  the  body  of  a  full-grown  man  may  be  taken 
at  70  kilogrammes  (154  lbs.). 


I.   General  Statistics 
Such  a  body  would  be  made  up  of  — 


Per  cent. 

lbs. 

.      .      .      .     41      . 

6.3 

.      .      .      .      16      . 

25 

IO.7 

28 

7 

Fat 

.      .      .       .       18      . 

Brain 

2 

3 

3 

10.7 

2 

.    .    .    .      7     . 

Blood1  

10.7 
x54 

of— 

100 

89 

....    42    . 

65 

The  solids  would  consist  of  the  elements  oxygen,  hydro- 
gen, carbon,  nitrogen,  phosphorus,  sulphur,  silicon,  chlorine, 
iodine,  fluorine,  potassium,  sodium,  calcium,  lithium,  mag- 

1  The  total  quantity  of  blood  in  the  body  is  calculated  at  about  A  to  A 
of  the  body  weight. 

555 


556  APPENDIX 

nesium,   iron,  manganese,   copper,  and  lead,   and  may  be 
arranged  under  the  heads  of — 

Proteids.  Carbohydrates.  Fats.  Minerals. 

Such  a  body  would  lose  in  24  hours  —  of  water,  about 
2,780  grammes  (6  lbs.  or  6  pints)  ;  of  other  matters,  about 
940  grammes  (2  lbs.),  which  would  contain  about  270-300 
grammes  (or  rather  more  than  ^  lb.)  of  carbon,  20  grammes 
(f  oz.)  of  nitrogen  and  30  grammes  (about  1  oz.)  of  mineral 
matters  (inorganic  salts). 

It  could  do  about  150,000  kilogramme-metres  (540  foot- 
tons  1)  of  work,  and  gives  off  as  much  heat  (2,300  kilogramme 
degree  units)  as  would  be  able  to  do  five  times  as  much 
work  again,  say  850,000  kilogramme-metres  (or  about  3,100 
foot-tons).  The  total  energy  expended  by  the  body  as 
heat  and  work  (calculated  entirely  as  work)  is  thus  about 
1,000,000  kilogramme-metres  (3,640  foot-tons),  of  which 
one-sixth  is  expended  as  work  and  five-sixths  as  heat. 

The  loss  of  substance  would  occur  through  various  organs 
and  to  the  respective  amounts  shown  in  the  table  on 
p.   293. 

The  gains  and  losses  of  this  body  would  be  about  as 
follows  :  — 

Creditor: — Solid  dry  food  .  .  600  grammes  (\\  lbs.) 
Oxygen  ....  640  "  (i|  "  ) 
Water 2,500        "         (5^   "   ) 

3,740  grammes  (8£  lbs.) 

Debtor :  —  Water 2,800  grammes  (6^  lbs.) 

Other  matters      .     .        940         "         (2     "    ) 

3,740  grammes  (8£  lbs.) 

1  A  foot-ton  is  the  equivalent  of  the  work  required  to  lift  one  ton  one 
fool  high. 


APPENDIX  557 

II.   Nutrition 

Such  a  body  would  require  for  daily  food,  carbon  270-300 
grammes,  nitrogen  20  grammes. 

Now  proteids  contain,  in  round  numbers,  about  15  per 
cent,  nitrogen  and  53  per  cent,  carbon,  while  carbohydrates 
and  fats  contain  respectively  40  per  cent,  and  80  per  cent, 
carbon.  Hence  the  necessary  amounts  of  nitrogen  and 
carbon,  together  with  the  other  necessary  elements,  might 
be  obtained  as  follows  (see  p.  295)  :  — 

Proteids ....     130  grms.  containing  20  grms.  nitrogen    70  grms.  carbon 
Carbohydrates     400     "  160      " 

Fats 50     "  "  40     "  " 

Minerals   ...       30     "  ■ 

Water    ....  2,500     " 


This  might  in  turn  be  obtained,  for  instance,  from  :  — 
Lean  meat 230  grammes  (|  lb.) 


Bread 480 

Potatoes 660 

Milk 500 

Fat 30 

Water 2,000 


(1  lb.) 

(ii  lb.) 

(t  P^t) 

(1  oz.) 
(4  pints) 


This  table,  however,  must  be  understood  as  being  intro- 
duced for  the  sake  of  illustration  only.  Many  other  similar 
tables  may  be  constructed  by  the  use  of  various  kinds  of 
food. 

III.   Circulation 

In  such  a  body  the  heart  would  beat  about  72  times  in  a 
minute  and  probably  drive  out  at  each  stroke  from  each 
ventricle  about  100  to  125  grammes  (6  to  7  cubic  inches  or 
3|  oz.)  of  blood. 


558  APPENDIX 

The  blood  would  probably  move  in  the  great  arteries  at 
the  rate  of  about  12  inches  (300  millimetres)  in  a  second: 
in  the  capillaries  at  the  rate  of  1-2  inches  (25-50  milli- 
metres) in  a  minute.  The  time  taken  up  in  performing  the 
complete  circuit  would  probably  be  a  little  less  than  30 
seconds. 

The  left  ventricle  would  probably  establish  a  blood-pres- 
sure in  the  aorta  equal  to  the  pressure  (per  square  inch)  of 
a  column  of  blood  about  7  or  8  feet  (2  metres)  in  height; 
or  of  a  column  of  mercury  6-7  inches  (150  millimetres)  in 
height. 

Sending  out  100  grammes  of  blood  at  each  stroke  against 
this  pressure  the  left  ventricle  does  100  x  2,000  gramme- 
millimetres  or  200  gramme-metres  of  work  at  each  stroke  ; 
in  24  hours,  at  72  strokes  per  minute,  the  total  work  done 
is  about  20,000  kilogramme-metres.  The  work  of  the  right 
ventricle  is  about  one-quarter  of  that  done  by  the  left,  since 
it  works  against  a  smaller  blood-pressure  in  the  pulmonary 
artery.  The  total  work  of  both  ventricles  is  therefore  about 
25,000  kilogramme-metres,  or  90  foot-tons. 

IV.   Respiration 

Such  a  body  would  breathe  about  1 7  times  a  minute. 

The  lungs  would  contain  of  residual  air  about  1,500  c.c. 
^100  cubic  inches),  of  supplemental  or  reserve  air  about 
1,500  c.c.  (100  cubic  inches),  of  tidal  air  500  c.c.  (30  cubic 
inches),  and  of  complemental  air  500  c.c.  (100  cubic  inches). 

The  vital  capacity  of  the  chest  —  that  is,  the  greatest 
quantity  of  air  which  could  be  inspired  or  expired  —  would 
be  about  3,500  c.c.  (230  cubic  inches). 

There  would  pass  through  the  lungs,  per  diem,  about 
10,000  litres  (350  to  400  cubic  feet)  of  air. 


APPENDIX  559 

In  passing  through  the  lungs,  the  air  would  lose  from  4  to 
6  per  cent,  of  its  volume  of  oxygen,  and  gain  4  to  5  per 
cent,  of  carbonic  acid. 

During  24  hours  there  would  be  consumed  of  oxygen 
about  450  litres  (16  cubic  feet)  or  640  grammes  (1^-  lb.)  ; 
there  would  be  produced  about  the  same  volume  (or  rather 
less)  of  carbonic  acid,  which  would  contain  about  225 
grammes  (8  oz.)  of  carbon.  During  the  same  time  about 
500  grammes  (1  pint  or  20  oz.)  of  water  would  be  given 
off  from  the  respiratory  organs. 

In  24  hours  such  a  body  would  vitiate  1,750  cubic  feet 
(1  cubic  foot  =  28.3  litres)  of  pure  air  to  the  extent  of  1  per 
cent,  or  17,500  cubic  feet  of  pure  air  to  the  extent  of  1  per 
1,000.  Taking  the  amount  of  carbonic  acid  in  the  atmos- 
phere at  3  parts,  and  in  expired  air  at  470  parts  in  10,000, 
such  a  body  would  require  a  supply  per  diem  of  more  than 
23,000  cubic  feet  of  ordinary  air,  in  order  that  the  surround- 
ing atmosphere  might  not  contain  more  than  1  per  1,000  of 
carbonic  acid  (when  air  is  vitiated  from  animal  sources  with 
carbonic  acid  to  more  than  1  per  1,000  the  concomitant 
impurities  become  appreciable  to  the  nose).  But  for  health, 
the  percentage  of  carbonic  acid  should  be  kept  down  to  half 
this  amount  or  .5  per  1,000,  so  that  the  body  should  be 
supplied  with  at  least  about  50,000  cubic  feet  of  fresh  air 
each  day.  A  man  of  the  weight  mentioned  (154  lbs.)  ought, 
therefore,  to  have  at  least  1,000  cubic  feet  of  well-ventilated 
space. 

V.   Cutaneous  Excretion 

Owing  to  its  excessive  variation  exact  figures  regarding 
cutaneous  excretion  are  of  very  little,  if  any,  value.  The 
body  mentioned  might,  however,  throw  off  by  the  skin  in  24 
hours  —  of  water   600   grammes   (20  oz.  or  1^  pint);   of 


560  APPENDIX 

solid  matters  12  grammes  (185  grains)  ;  of  carbonic  acid 
10  grammes  (150  grains). 

VI.  Renal  Excretion 

Such  a  body  would  pass  by  the  kidneys  in  24  hours  —  of 
water  about  1,500  grammes  or  cubic  centimetres  (53  oz. 
or  3  pints)  ;  of  urea  about  33  grammes  (500  grains  or  1^ 
oz.),  and  about  the  same  quantity  of  other  solid  matters. 

VII.  Nervous  Action 

A  nervous  impulse  travels  along  a  nerve  at  the  rate  of 
about  90  feet  in  a  second  in  the  frog,  and  cf  about  100  feet 
a  second  in  man ;  but  the  rate  in  man  varies  very  much 
according  to  circumstances. 

VIII.   Histology 

The  following  are  some  of  the  most  important  histological 
measurements  :  — 

Red  blood-corpuscles,  breadth  3-2V0  of  an  inch,  or  7/u.  to 

8/A. 

White   blood-corpuscles,   breadth   ^V^   °f  an   incn>   or 

I  Oft. 

Striated  muscular  fibre  (very  variable),  breadth  -^\-§  of 
an  inch,  or  60/x ;  length  1^-  inch,  or  30  to  40  millimetres. 

Non-striated  muscular  fibre  (variable),  breadth  4-,^  of 
an  inch,  or  6/u, ;  length  2x0  °f  an  incn>  or  100/4. 

Nerve-fibre  (very  variable),  breadth  jyvws  to  TsVo  °f 
an  inch,  or  2/jl  to  16/x. 

Nerve-cells  (of  spinal  cord)  excluding  processes,  breadth 
,')()  to  T7T  or  more  °f  an  inch,  50/x  to  140/x  or  more. 


APPENDIX  561 

White  fibres  of  connective  tissue,  breadth  2TQo~5  °f  an 
inch,  or  i/a. 

Superficial  cells  of  epidermis,  breadth  j^Vo  °f  an  mcn>  or 

Capillary  blood-vessels  (variable)  width  .i^Q0  to  ^  ^  0  of 
an  inch,  or  7/x,  to  12/t. 

Cilia,  from  the  wind-pipe,  length  goVo  °f  an  mcn  or 
8/x. 

Cones  in  the  yellow  spot  of  the  retina,  width  ygVo  °f 
an  inch,  or  2/x. 


SO 


INDEX 


Abdomen,  6. 

Abduction  of  limb,  353. 

Absorption    of    digested    food,   249, 

274,  288. 
Accelerator  nerves,  99. 
Accommodation    of    the    eye,   433 ; 

diagram    of,   435,  436;    limits   of, 

437- 

Acetabulum,  352. 

Acid,  carbonic,  see  Carbonic  acid ; 
glycocholic,  240;  hydrochloric,  in 
gastric  juice,  270;  sarcolactic,  318, 
319,  322;  taurocholic,  240;  uric, 
209. 

Action,  reflex,  see  Reflex  action ; 
spontaneous,  367. 

Adam's  apple,  357. 

Adduction  of  limb,  353. 

Adenoid  tissue,  53,  116. 

Adipose  tissue,  53 ;  figure  of,  54. 

Afferent  nerve,  367,  500. 

After-images,  negative,  457. 

Air,  amount  of,  respired,  172  ; 
changes  of,  in  respiration,  174; 
complemental,  172 ;  composition  of 
atmospheric,  4  ;  composition  of  in- 
spired and  expired,  174;  expired, 
temperature  of,  174;  residual,  172; 
stationary,  173 ;  supplemental,  172  ; 
tidal,  172;  when  injurious,  187, 
191. 

Air-cells  in  lungs,  155,  173. 

Albumin  as  food,  251,  298. 

Alimentary  canal,  7,249;  figure  of, 
277 ;  muscle  of,  325. 

Alimentary  organs,  21. 

Alimentation,  249. 


Alveolus,  of  gland,  263;  of  lung,  155, 
173;  of  tooth,  255. 

Amoeba,  colourless  corpuscle  com- 
pared to,  128. 

Amoeboid  movement,  128,  308. 

Ampulla  of  semicircular  canals,  395. 

Aniylopsin,  284. 

Anus,  276. 

Aorta,  63;  abdominal,  193. 

Appendix,  vermiform,  276. 

Aqueous  humour,  424. 

Arachnoid  membrane,  477. 

Areolar  tissue,  112. 

Arm,  figure  of  bones  of,  17,  326. 

Artery  (or  arteries),  22;  and  vein, 
56 ;  contractility  of,  58 ;  coronary, 
65;  elasticity  of,  57,  81,  84,  86; 
function  of  muscle  of,  59 ;  hepatic, 
65,  235 ;  iliac,  193 ;  nervous  con- 
trol of,  90;  peripheral  resistance 
in,  81,  83,  84,  109;  pressure  in,  83; 
pulmonary,  63;  renal,  204;  struc- 
ture of,  57 ;  figure  showing  struc- 
ture of,  58,  60 ;  tone  of,  93  ;  working 
of,  80. 

Articular  cartilages,  17,  346. 

Articulations  or  joints,  345. 

Arytenoid  cartilages,  358. 

Asphyxia,  186. 

Aspirate  sounds,  364. 

Association,  law  of,  547. 

Association  nerve-fibres,  553. 

Astragalus,  355. 

Atlas,  348. 

Auditory  organs,  392. 

Auditory  epithelium,  393. 

Auditory  sensorium,  414. 


563 


564 


INDEX 


Auditory  spectra,  463. 
Auricle  of  heart,  68. 
Auriculo-ventricular  apertures,  69. 
Automatism  of  the  respiratory  centre, 

181. 
Axis,  348. 

Axis-cylinder,  485,  487;  process,  481. 
Axon,  481. 

Ball-and-socket  joints,  347,  352,  354. 
Basilar  membrane,  404;  function  of, 

417. 
Beat  of  heart,  75. 
Bile,  239;    colour  of,  131;    function 

of,  286;  secretion  of,  285. 
Bile  duct,  figure  showing  origin  of, 

239- 

Bile-pigments,  240,  285. 

Bile-salts,  240,  285. 

Bilirubin,  240. 

Biliverdin,  240. 

Bladder,  8,  201. 

Blastomeres,  33. 

Blind  spot,  449,  450. 

Blister,  how  formed,  36. 

Blood,  119;  arterial  and  venous  com- 
pared, 150,  186;  changes  of,  in 
lungs,  178 ;  circulation  of,  22,  55 ; 
clotting  of,  120,  136;  colour  of 
arterial  and  venous,  149 ;  composi- 
tion of,  132;  distribution  of,  140; 
functions  of,  141 ;  gases  of,  148 ; 
microscopic  examination  of,  119; 
physical  qualities  of,  131 ;  quantity 
of,  140;  sources  of  loss  and  gain 
to,  193,  249;  specific  gravity  of, 
131;  temperature  of,  132;  trans- 
fusion of,  142. 

Blood-capillaries,  56. 

Blood-corpuscles,  120;  colourless,  32, 
121,  126,  129;  of  embryos,  128; 
figure  of,  121,  127;  migration  of, 
129;  movement  of,  107,  126;  num- 
ber of,  131 ;  origin  and  fate  of,  130 ; 
properties  of,  131 ;  red,  121,  122. 

Blood-crystals,  126;  figure  of,  125. 

Blood-flow,  rate  of,  89. 


Blood-plasma,  120,  134. 

Blood-platelets,  130. 

Blood-pressure,  83 ;  in  capillaries,  146. 

Blood-strum,  137. 

Blood-vessels,  general  arrangement 
of,  61,  62. 

Blushing,  91. 

Body,  chief  tissues  of,  34 ;  composi- 
tion of,  292,  555 ;  daily  income  of, 
295;  daily  outgo  from,  293;  de- 
composition of,  28;  diagrammatic 
section  of,  9 ;  figure  illustrating 
erect  posture  of,  18 ;  general  build 
of,  6;  movements  of,  353;  oxida- 
tion in,  24;  table  of  gains  and  losses 
of,  556;  temperature  of,  227;  tem- 
perature of,  and  blood  supply,  94; 
work  of,  1,  305,  556. 

Bone  (or  bones),  325;  composition 
of,  14 ;  development  of,  335 ;  of 
ear,  407,  412;  ethmoid,  389;  figure 
showing  development  of,  338; 
figure  showing  structure  of,  326, 
332,  333 ;  hyoid,  357 ;  marrow  of, 
329;  number  of,  14;  orbicular, 
408;  periosteal,  338;  petrous,  393; 
spongy,  338 ;  structure  of,  328 ; 
temporal,  393;  turbinal,  389. 

Bone  corpuscles,  334. 

Bony  tissue,  330. 

Brain,  8,  475 ;  anatomy  of,  517 ;  basal 
view  of,  520;  and  consciousness, 
544;  effects  of  destruction  of,  in 
frog,  99, 508 ;  horizontal  section  of, 
527 ;  lobes  of,  547 ;  median  view  of, 
522;  membranes  of,  528;  minute 
structure  of,  529 ;  paths  of  conduc- 
tion in,  551;  reflex  action  of,  545; 
side  view  of,  518. 

Bread  as  food,  299. 

Breathing,  see  Respiration. 

Broca's  convolution,  549. 

Bronchi,  155,  157,  158. 

Bronchial  tubes,  155,  157,  158. 

Bronchioles,  155. 

Brunner,  glands  of,  280. 

Buccal  glands,  262. 


INDEX 


565 


Buffy-coat,  137. 

Bulb,  517,519,521,  538. 

Bursas,  327. 

Caecum,  276;  figure  of,  276. 

Calamus  scriptorius,  521. 

Calorie,  305. 

Camera  obscura,  431. 

Canal,  alimentary, 7,  249;  alimentary, 
figure  of,  277;  central,  of  spinal 
cord,  478;  Haversian,  329;  semi- 
circular, 395,  421,  541 ;  spinal,  7. 

Canaliculi  in  bone,  333. 

Cancelli  of  articular  end  of  bone,  329. 

Cancellous  or  spongy  tissue  of  bone, 

325- 

Capillaries,  22;  condition  of  walls  of, 
109 ;  figure  showing  distribution  of, 
104,  106;  pressure  in,  146;  pulmo- 
nary, 153 ;  structure  of,  55 ;  figure 
showing  structure  of,  55. 

Capsule,  of  cartilage,  45 ;  internal, 
552;  of  joints,  352;  Malpighian, 
204,  206,  211,  212. 

Carbohydrate,  4 ;  absorption  of,  289, 
290;  digestion  of,  267,  282,  287  ;  as 
food,  250,  251,  299,  301 ;  heat-equiv- 
alent of,  306. 

Carbon,  amount  eliminated  by  lungs, 
175 ;  daily  waste  of,  294. 

Carbonic  acid,  3;  in  air,  4,  174;  in 
blood,  132,  149,  152;  effect  of,  on 
blood-corpuscles,  151 ;  effect  of,  on 
organism,  187;  in  lymph,  143;  in 
urine,  209. 

Carbonic  oxide,  effect  of,  187. 

Cardiac  aperture,  269. 

Cardiac  cycle,  76. 

Cardiac  impulse,  81. 

Cardiac  muscle,  74. 

Cardiac  nerves,  98. 

Cardio-inhibitory  centre,  101,  538; 
diagram  of,  102. 

Cartilage,  articular,  17,  346;  aryte- 
noid, 358;  calcified,  336;  cricoid, 
357;  development  of,  47  ;  elastic  or 
yellow  fibro-,  46 ;  figure  of  elastic, 


46;  figure  of  hyaline,  44,45  ;  figure 
of  white  fibro-,  47  ;  hyaline,  42;  in- 
ter-articular, 346 ;  structure  of,  42  ; 
thyroid,  357  ;  white  fibro-,  46. 

Cartilage  corpuscles,  44. 

Casein  as  food,  251. 

Cells,  32;  amoeboid,  308;  central,  of 
gastric  glands,  270 ;  ciliated,  157 ; 
differentiation  of,  33 ;  gustatory, 
385;  parietal,  of  gastric  glands, 
270;  of  Purkinje,  531;  secreting, 
199,  207,  219,  238,  263,  271,  282; 
figure  of  secreting,  208,  264,  265, 
271,  283  ;  wandering,  51. 

Cement  of  teeth,  257. 

Centre,  cardio-inhibitory,  101,  538 ; 
of  ossification,  336;  respiratory, 
180;  vaso-motor,  94,  514,  538. 

Cerebellum,  518,  523  ;  figure  illustrat- 
ing structure  of,  531 ;  functions  of, 
540;  peduncles  of,  525;  structure 
of,  530. 

Cerebral  hemispheres,  518,  519,  522; 
diagram  of  localization  in,  550; 
diagrams  of  surface  of,  548  ;  func- 
tions of,  542;  localization  of  func- 
tion in,  547. 

Cerebro-spinal  nervous  system,  8, 
475 ;  membranes  of,  476. 

Cerebrum,  518 ;  diagrams  of  locali- 
zation in,  550 ;  diagrams  of  surface 
of,  548  ;  functions  of,  542 ;  localiza- 
tion of  function  in,  547. 

Chest,  160;  figures  of,  66  157,  161 
163,  171. 

Cholesterin,  240;  285. 

Chondrin,  43. 

Chorda  tympani  nerve,  96,  266. 

Chordae  tendineae,  69  ;  action  of,  78. 

Choroid  coat,  425. 

Choroid  plexuses,  529. 

Chyle,  112,  286,  289;  compared  with 
lymph,  144;  receptacle  of,  112. 

Chyme,  274. 

Cilia,  41,  308  ;  action  of,  157,  308. 

Ciliary  muscle,  427. 

Ciliary  processes  of  choroid,  425. 


566 


INDEX 


Ciliated  cells,  157;  figure  of,  159, 
308. 

Ciliated  epithelium,  41. 

Circulation  of  the  blood,  22, 55  ;  cap- 
illary, 105 ;  figure  of  capillary,  104, 
106,  158 ;  figure  showing  course 
of,  62 ;  influence  of  respiration  on, 
188  ;  in  the  kidney,  205,  208 ;  portal, 
65 ;    proofs   of,    103 ;    statistics   of, 

557- 

Circulatory  organs,  22,  55. 

Circumduction  of  limb,  353. 

Clavicle,  17. 

Coagulation  of  blood,  120,  136. 

Coccyx,  15. 

Cochlea,  400;  bony,  405;  canal  of, 
401 ;  diagram  of,  401,  403 ;  mem- 
branous, 399. 

Cold,  sensations  of,  378. 

Colloids,  146. 

Colon,  276. 

Colour,  qualities  of,  454;    sensations 

of,  453- 

Colours,  complementary,  455 ;  pri- 
mary, 456. 

Colour-blindness,  457. 

Columnse  carneae,  69. 

Complemental  air,  172. 

Cones  of  retina,  443,  452. 

Conjunctiva,  375,  440. 

Connective  tissue,  11,  49;  adeno'd, 
retiform,  or  lymphoid,  53, 116;  are- 
olar, 49,  112;  development  of,  53; 
elastic,  52;  fatty  or  adipose,  53; 
fibrous,  52;  structure  of,  49. 

Connective  tissue  corpuscles,  50; 
figure  of,  52. 

Connective  tissue  fibres,  figure  of,  49, 

50,  51- 
Consciousness  and  brain,  544. 
Consciousness  and  sensations,  369. 
Consciousness,  states  of,  370. 
Consonants,  364. 
Contact,  sense  of,  382. 
Contractility,  of  colourless  corpuscle, 

127 ;  of  muscle  fibre,  317,  320. 
Contraction,  peristaltic,  262,  325. 


Convolution,   Broca's,   549;    of  cer- 
ebrum, 547. 
Coordinating  action  of  nervous  sys- 
tem, 25. 

Coordination  of  movements,  540. 

Cornea,  424. 

Cornua  of  spinal  cord,  480. 

Coronary  arteries,  65. 

Coronary  vein,  65. 

Corpora  quadrigemina,  519,  523,  526. 

Corpora  striata,  528. 

Corpus  callosum,  521,  523. 

Corpuscles,  of  blood,  see  BLood-cor- 
puscles ;  of  bone,  334 ;  of  cartilage, 
44;  of  connective  tissue,  50 ;  of  the 
spleen,  245;  tactile,  217,  374. 

Corresponding  points,  440, 472. 

Cortex,  of  cerebellum,  530;  of  cer- 
ebrum, 529,  532;  of  cerebrum, 
figure  illustrating  structure  of,  531, 
533;  of  cerebrum,  localization  of 
function  in,  547 ;  of  kidney,  204. 

Corti,  organ  of,  402;  organ  of,  dia- 
gram of,  403 ;  organ  of,  function 
of,  417 ;  rods  of,  404. 

Coughing,  170. 

Cranial  nerves,  475,519,535 ;  diagram 
of,  536. 

Creatin,  319. 

Cretinism,  247. 

Cribriform  plate,  389. 

Cricoid  cartilage,  357. 

Crista  acustica,  396 ;  diagram  of,  398. 

Cruracerebri,  510,  522,  526. 

Crystalline  lens,  424,  437. 

Crystalloids,  146. 

Death,  and  life,  25 ;  local  and  general, 
26 ;  modes  of,  27. 

Death-stiffening,  318. 

Decomposition  of  the  body,  28. 

Decussation,  of  pyramids,  539;  sen- 
sory, 539. 

Defsecation,  511. 

Degeneration,  ascending,  512;  de- 
scending, 512;  in  spinal  cord,  dia- 
gram of  tracts  of,  513,  514. 


INDEX 


567 


Degeneration-method,  499. 

Delirium  tremens,  463. 

Delusions,    of   judgment,   461,  462, 

465 ;    optical,  466. 
Dendrite,  481. 
Dental  groove,  257. 
Dental  papilla,  258. 
Dental  pulp,  255,  257. 
Dentine,  256. 

Dentition,  milk,  260;  permanent,  260. 
Dermis,  10,  11,  35,  215. 
Dextrose,  251. 
Diabetes,  284,  539. 
Diaphragm,  7,  166;    action  of,  167; 

of   camera,   431;     figure   of,    167; 

pillars  of,  166. 
Diaphysis,  336. 
Diastole  of  heart,  75. 
Diet,  295,  557;    economy  of  mixed, 

297 ;    effect  of,  on  red  corpuscles, 

133- 

Differentiation  of  primitive  cells,  33. 

Diffusion,  144  ;  figure  illustrating,  145. 

Digestion,  249;  intestinal,  285;  pur- 
pose and  means  of,  252. 

Digits,  7. 

Distance,  judgment  of,  468. 

Drinking,  mechanism  of,  262. 

Drum  of  ear,  406. 

Duct,  hepatic,  235;  lachrymal,  441 ; 
pancreatic,  285;  thoracic,  in. 

Ductless  glands,  201. 

Duodenum,  274  ;  figure  of,  245. 

Dura  mater,  477. 

Dyspnoea,  185. 

Ear,  392 ;  figure  of,  404, 407 ;  external, 
394,  410;  internal,  394;  transmis- 
sion of  sound  waves  to,  410;  mid- 
dle, 406;   middle,  diagram  of,  405. 

Education  and  nervous  system,  547. 

Efferent  nerve,  367,  500. 

Egg,  32;  diagram  of,  32;  segmenta- 
tion of,  33  ;  diagram  of  segmenta- 
tion of,  34. 

Electric  shock,  effect  of,  on  muscle, 
322. 


Electrical  fishes,  500. 

Electricity,    of    contracting    muscle, 

322 ;    of  nerve,  502. 
Embryo,  128. 
Emulsification,  2860 
Emulsion,  284. 
Enamel,  257. 

End-bulbs,  217,  375 ;  figure  of,  375. 
Endocardium,  75. 
Endolymph,  393. 

End-organ,  mode  of  action  of  audi- 
tory, 415 ;  motor,  487. 
End-plate,  487. 
Energy,  daily  output  of,  305  ;  income 

and   expenditure   of,   303 :    source 

of  vital,  304. 
Enzymes,  267,  268. 
Epidermis,  10,  215;    growth  of,  38; 

structure  of,  35,  217. 
Epiglottis,  155,  254. 
Epinephrin,  248. 
Epiphyses,  336,  337. 
Epithelium,    11;     auditory,   393;    o: 

blood-vessels,  etc.,  42 ;  ciliated,  41 ; 

figure  of  cells  of,  38 ;    of  mucous 

membrane,    41 ;     olfactory,    389 ; 

secreting,  199. 
Equilibrium,  maintenance  of  bodily, 

422. 
Erect  position,  how  maintained,  17, 

18. 
Eustachian  tube,  254,  406;  function 

of,  421. 
Excretion,  4 ;    oxygen  in,  4 ;  statistics 

of   cutaneous,    559;     statistics    of 

renal,  560. 
Excretory  organs,  23,  194, 
Expiration,  168. 

Expired  air,  composition  of,  174. 
Explosive  sounds,  365. 
Extension  of  limb,  353. 
Eye,  423;    accommodation  of,  433; 

movements     of,    439  ;    protective 

appendages  of,  440 ;    structure  of, 

423 ;  as  water  camera,  428. 
Eyeball,  424;  diagram  of  muscles  oi, 

439 ;  section  of,  425. 


568 


INDEX 


Eyelashes,  440. 

Eyelids,  440;   muscles  of,  441. 

Face,  cavity  of  the,  8 ;  figure  showing 
section  of,  156,  253. 

Faeces,  22,  252. 

Fainting,  92. 

Fascia,  312. 

Fasciculi  of  muscle,  312. 

Fat,  absorption  of,  288,  290;  in  blood- 
corpuscles,  129 ;  in  chyle,  144 ;  di- 
gestion of,  284,  286 ;  as  food,  250, 
251,299,301 ;  heat-equivalent  of,  306. 

Fat-cells,  53 ;  figure  of,  54. 

Fatigue,  one  cause  of,  177  ;  of  retina, 

456- 

Fauces,  254. 

Femur,  figure  showing  structure  of, 
326. 

Fenestra  ovalis,  406,  407. 

Fenestra  rotunda,  401,  406,  411 

Ferments,  140,  267,  268. 

Fever,  temperature  in,  233. 

Fibres,  association,  553;  collagenous, 
50 ;  elastic,  50 ;  white,  50. 

Fibrin,  136  ;  figure  of,  135. 

Fibrin  ferment,  140. 

Fibrinogen,  135,  139,  143. 

Filtration,  144. 

Fishes,  electrical,  500. 

Fissure,  anterior,  of  spinal  cord, 
478 ;  calcarine,  547  ;  parieto-occipi- 
tal,  547;  posterior,  of  spinal  cord, 
478 ;  of  Rolando,  525,  547 ;  of  Syl- 
vius, 525,  547. 

Flexion  of  limb,  353. 

Fluid,  cerebro-spinal,  477. 

Focus  of  lens,  430. 

Food,  4,  250 ;  changes  of,  in  intestine, 
285;  proteid  and  carbohydrate,  4; 
as  source  of  energy,  304  ;  waste 
made  good  by,  249. 

Food-stuffs,  250  ;  accessory,  303 ; 
effects  of  the  several,  300 ;  essen- 
tial, 303;  as  heat-producers  and 
tissue-formers,  302  ;  nitrogenous, 
250;  non-nitrogenous,  251. 


Foot  as  lever,  342. 
Foot-pound,  304. 

Foramen,  of  Magendie,  529 ;  of  Mon- 
ro, 524;  occipital,  352. 
Foramina,  intervertebral,  478. 
Forearm,  figure  of  bones  of,  351. 
Fore-brain,  519. 
Form,  judgment  of,  470,  471. 
Fornix,  523. 

Gall-bladder,  235 ;  figure  of,  269 

Galvanometer,  503. 

Ganglia,  in  heart,  98 ;  spinal,  479 , 
spinal,  structure  of,  494;  sympa- 
thetic, 7,  475,  514. 

Ganglion-cell,  figure  of,  494. 

Gases,  exchange  of,  in  lungs,  178; 
of  inspired  and  expired  air,  174; 
partial  pressure  of,  177. 

Gastric  glands,  270;  figure  of,  271. 

Gastric  juice,  270;  action  of,  271; 
secretion  of,  271. 

Gelatine,  of  bone,  11,330;  as  food, 
251. 

Gland  (or  glands) ,  in  general,  199 ;  of 
Brunner,  280;  buccal,  262;  changes 
in,  in  secretion,  263,  271,  281 ;  duct- 
less, 201;  ducts  of,  199;  figure  show- 
ing structure  of,  200 ;  gastric,  270; 
figure  of,  271;  lachrymal,  441;  of 
Lieberkuhn,  279;  lymphatic,  in; 
lymphatic,  figure  of,  115;  Meibo- 
mian, 440;  parotid,  262;  pineal, 
523;  saccular,  199;  salivary,  262; 
sebaceous,  223  ;  secreting,  199 ; 
simple  and  compound,  199;  sub- 
maxillary, 262;  sublingual,  262; 
sweat,  217;  thymus,  246;  thyroid, 
246;  tubular,  199. 

Glomerulus  of  kidney,  204 ;  figure  of, 

205,  206,  209. 
Glottis,  155,  254,  357;  figure  of,  359. 
Gluten  as  food,  251. 
Glycocholic  acid,  240. 
Glycogen,  in  blood-corpuscles,  129; 

in  liver,  234,  242;  in  muscle,  319. 
Goitre,  247. 


INDEX 


569 


irey  matter,  of  brain,  526,  529;  of 
spinal  cord,  480;  of  spinal  cord, 
cells  of,  491 ;  of  spinal  cord,  figure 
of  cells  of,  492. 

Gristle,  12. 

Gullet,  8. 

Gum  of  mouth,  255. 

Gustatory  cells,  385. 

Gustatory  nerve,  385. 

Gyri,  547. 

Hcematin,  124,  131. 

Haemoglobin,  124,  151 ;  crystals  of, 
125  ;  in  muscle,  320. 

Hair,  221 ;  figure  of,  221,  222,  223. 

Hairs,  auditory,  397. 

Hair-cells,  of  cochlea,  404;  of  organ 
of  Corti,  418. 

Hand,  movements  of,  351. 

Harvey,  discoverer  of  the  circulation, 
103. 

Haversian  canals,  329. 

Haversian  system,  331. 

Head,  movements  of,  349,  350. 

Hearing,  sense  of,  392. 

Heart,  22,  66 ;  beat  of,  75 ;  diagram 
showing  action  of,  78 ;  figures  of, 
64,  68,  70,  71,  72,  73  ;  ganglia  in, 
98 ;  nervous  control  of,  98 ;  pal- 
pitation of,  103 ;  sounds  of,  82 ; 
structure  of,  66,  74;  valves  of,  69; 
valves  of,  action  of,  76. 

Heat,  227. 

Heat,  developed  by  contracting  mus- 
cle, 322,  323;  loss  of,  229;  mechan- 
ical equivalent  of,  305  ;  production 
of,  232;   unit  of,  305. 

Heat  spots  and  cold  spots,  380;  dia- 
gram of,  379. 

Hiccough,  170. 

Hilus  of  kidney,  201. 

Hind-brain,  519. 

Hinge-joints,  348,  352. 

Hip  joint,  352;  figure  of,  347. 

Histology,  30;  statistics  of,  560. 

Homoiomera,  30. 

Humerus,  350. 


Humours  of  the  eye,  424. 
Hyoid  bone,  328,  357. 
Hypoglossal  nerve,  538. 

Ileo-caecal  valve,  274. 

Ileum,  274. 

Ilium,  17. 

Incus,  408,  412. 

Inflammation,  107. 

Infundibulum    of  a  bronchial   tube, 

155- 

Innervation,  475. 

Innominate  bone,  17. 

Insertion  of  muscle,  326. 

Inspiration,  168. 

Inspired  air,  composition  of,  174. 

Intelligence,  seat  of,  in  cerebrum,  542. 

Intercellular  substance,  32,  42,  44. 

Intercostal  muscles,  162 ;  diagram  of, 
165  ;  figure  of,  164. 

Intercostal  nerves,  180. 

Internal  capsule,  552. 

Internal  secretion,  201 ;  by  liver,  244  ; 
by  thyroid  body,  246 ;  by  supra- 
renal bodies,  248. 

Intestinal  juice,  282. 

Intestines,  absorption  from,  287,  288  ; 
arrangement  of,  274 ;  changes  of 
food  in,  285  ;  structure  of,  278. 

Iodo-thyrin,  247. 

Iris  of  the  eye,  421, 426, 432 ;  arrange- 
ment of  muscle  in,  324 ;  figure  of, 
428. 

Ischium,  17. 

Jejunum,  274. 

Joints,  345-353  ;  ball-and-socket,  347, 
352>  354;  hinge,  348;  imperfect, 
345;    perfect,  346;   pivot,  348. 

Judgment,  of  changes  of  form,  471 ; 
delusive,  461,  462,  465  ;  of  distance, 
468;  of  form,  470,  471 ;  of  size,  468; 
of  solidity,  473. 

Jumping,  mechanics  of,  356. 

Kidneys,  8,  201;  figure  of,  202,  204; 
figure  showing  cells  of,  208;  figure 


57° 


INDEX 


showing  circulation  in,  206,  209 ; 
function  of,  23,  211 ;  lungs  and  skin 
compared,  226;  and  skin,  212;  sta- 
tistics of  excretion  by,  560 ;  struc- 
ture of,  203;  types  of  cells  in,  206. 

Kilogramme-metre,  305. 

Kinetoscope,  451,  472. 

Knee-joint,  diagram  of,  345. 

Labyrinth,  bony,  394,  405;  mem- 
branous, 393,  395;  membranous, 
diagram  of,  396. 

Lachrymal  gland,  441. 

Lachrymal  sac,  441. 

Lacteals,  112,  144,  280. 

Lacunae  of  bone,  332. 

Lamellae,  of  cerebellum,  530;  of  bony 
tissue,  331. 

Lamina,  spiral,  399. 

Larynx,  155;  mechanism  of,  356; 
figure  of,  357,  358,  361 ;  figure  illus- 
trating action  of,  363. 

Law  of  association,  547. 

Leg,  figure  of  bones  of,  16. 

Lens,  crystalline,  424,  437. 

Lenses,  properties  of,  429. 

Leucin,  214,  284,  287. 

Leucocytes,  116,  143. 

Levatores  costarum,  165. 

Levers,  bones  as,  341-345  ;  classes  of, 
341 ;  figure  of,  342. 

Lieberkiihn,  glands  of,  279. 

Life,  and  death,  25;  tripod  of,  27. 

Ligaments,  17,  347;  capsular,  352; 
check,  352;  crucial,  352;  lateral, 
352;   round,  352;    suspensory,  425. 

Light,  sensations  of,  448,  451. 

Liver,  8,  233  ;  figure  of,  234,  236,  237  ; 
function  of,  214,  239 ;    structure  of, 

233- 
Liver-cells,  236,  238;    figure  of,  236, 

239- 

Lobes,  of  brain,  525;  of  liver,  235. 
Lobes  and  lobules  of  the  lungs,  155. 
Lobes,  olfactory,  535. 
Locomotion,  mechanics  of,  354. 
Longsightedness,  438. 


Lungs,  8,  155  ;  amount  of  waste  leav- 
ing, 175  ;  anatomy  of,  155  ;  elastic- 
ity of,  162;  figure  of,  66,  68;  figure 
showing  structure  of,  158  ;  function 
of,  23 ;  kidneys,  and  skin  com- 
pared, 226. 

Lymph,  no,  142 ;  composition  of, 
143 ;  functions  of,  147 ;  mode  of 
formation  of,  144;  movements  of, 
117. 

Lymph-channel,  116. 

Lymph-corpuscles,  143. 

Lymph-sinus,  116. 

Lymph-spaces,  114. 

Lymphatic  glands,  in;  figure  of, 
115;  function  of,  115;  structure 
of,  115. 

Lymphatic  system,  109. 

Lymphatic  vessels,  109,  in,  114;  gen- 
eral arrangement  of,  109 ;  figure  of 
course  of,  111;  figure  of  origin  of, 
114;  origin  of,  112;  structure  of,  112. 

Lymphoid  tissue,  53,  116. 

Macula  acustica,  396. 

Macula  lutea,  442,  448;    section  of, 

447- 

Magendie,  foramen  of,  529. 

Malleus,  407,  412. 

Malpighi,  network  of,  36. 

Malpighian  capsule,  204,  206;  figure 
of,  205,  206;  function  of,  211,  212. 

Malpighian  layer  of  epidermis,  36, 
217. 

Maltose,  267. 

Marrow  of  bone,  325,  329;  red  cor- 
puscles formed  in,  130. 

Mastication,  260. 

Matrix  of  cartilage,  42,  44. 

Meat  as  food,  299. 

Meatus  of  ear,  406. 

Mechanical  equivalent  of  heat,  305. 

Medulla,  of  bone,  329;  of  kidney, 
204;  of  lymphatic  gland,  116;  of 
nerve-fibre,  485  ;  oblongata,  517 ; 
oblongata,  functions  of,  538. 

Meibomian  glands,  440. 


INDEX 


57i 


Membrane,    arachnoid,   477 ;    base-  [ 

ment,  199 ;    basilar,  404,  417  ;    mu-  ! 

cous,    11,    41;     of   Reissner,  402;; 

serous,  67  ;  synovial,  17,  346. 
Membranous     labyrinth,    393,    395; 

diagram  of,  396. 
Mesentery,  276  ;  figure  of,  278. 
Metabolism,  213,  note. 
Micro-millimetre,  40. 
Micturition,  511. 
Mid-brain,  519. 
Milk  as  food,  300. 
Milk-teeth,  258,  260. 
Mitral  valve,  69. 
Monro,  foramen  of,  524. 
Mouth,  252;   figure  of  section  of,  156, 

253- 

Movements,  amoeboid,  308 ;  of  the 
body,  353  ;  mechanics  of,  341 ;  co- 
ordination of,  540. 

Mucin,  240. 

Mucous  membrane,  11,41. 

Mucus,  11. 

Muscle  (or  muscles),  attached  to  def- 
inite levers,  325 ;  not  attached  to 
solid  levers,  324  ;  biceps,  326,  353  ; 
biceps,  figure  of,  327 ;  capillaries 
of,  figure  of,  313 ;  changes  of  a  con- 
tracting, 322;  chemistry  of,  318; 
ciliary,  427 ;  colour  of,  319 ;  con- 
traction of,  12,  320,  323;  crico- 
thyroid, 358  ;  death  of,  318  ;  devel- 
opment of,  316;  digastric,  328;  of 
eyeball,  439;  external  rectus,  537; 
figure  showing  fasciculi  of,  312; 
gastrocnemius,  320  ;  hollow,  324  ; 
insertion  of,  326;  intercostal,  162; 
intercostal,  diagram  of,  165  ;  inter- 
costal, figure  of,  164 ;  kinds  of,  324 ; 
lateral  crico-arytenoid,  360;  levator 
of  eyelid,  440;  levatores  costarum, 
165 ;  obliqui  of  eye,  439 ;  orbicu- 
laris, 440;  as  organ  and  as  tissue, 
311;  origin  of,  326;  papillary, 
69;  plain,  310;  posterior  arytenoid. 
360 ;  posterior  crico-arytenoid,  360 ; 
recti  of  eye,  439  ;   rectus  abdominis, 


343;  rectus  femoris,  343;  scaleni, 
165;  smooth,  310;  sphincter,  203; 
stapedius,  410,  420;  striated,  311; 
superior  oblique,  327,  537;  tensor 
tympani,  410,  420;  tetanic  contrac- 
tion of,  323,  thyro-arytenoid,  360; 
triceps,  354  ;  of  tympanum,  413  ;  of 
tympanum,  function  of,  420;  un- 
striated,  310. 

Muscle-fibre,  cardiac,  74;  contrac- 
tility of,  317,  320;  size  of,  313  ;  stri- 
ated, 313;  striated,  figure  of,  314, 
316;  unstriated,  310. 

Muscle-nerve  preparation,  321. 

Muscle-plasma,  318. 

Muscle-serum,  318. 

Muscular  sense,  383. 

Muscularis  mucosae,  280. 

Musical  sounds,  415. 

Myelin,  485. 

Myosin,  318;  as  food,  251. 

Myosinogen,  319. 

Nails,  219 ;  figure  showing  structure 
of,  220. 

Nares,  posterior,  388. 

Nasal  cavity,  figure  of,  388,  390. 

Nearsightedness,  438. 

Nerve  (or  nerves),  476;  abducens, 
536;  accelerator,  99;  afferent,  367, 
500;  auditory,  393,  402,  422,  537, 
542;  cardiac,  98  ;  chorda  tympani, 
96,  266  ;  cochlear,  422,  542 ;  cranial, 
475-  5IQ.  535  ;  cranial,  diagram  of, 
536;  efferent,  367,  500;  electrical 
properties  of,  502;  facial,  537  ;  glos- 
sopharyngeal, 385,  537 ;  gustatory, 
385  ;  hypoglossal,  538  ;  inhibitory, 
500;  intercostal,  181;  medullated, 
484;  motor,  367,500;  motor  oculi 
or  oculo-motor,  432,  536  ;  olfactory, 
535;  optic,  423,  535 ;  phrenic,  181; 
physiological  properties  of,  499' 
pneumogastric,  98, 182, 538 ;  sciatic, 
321;  secretory,  226,  500;  sensory, 
367,  500;  spinal,  475,  478;  spinal, 
roots  of,  478,  495  ;  spinal  accessory, 


572 


INDEX 


538 ;  structure  of,  483 ;  superior 
laryngeal,  184 ;  sympathetic,  92, 
515;  trigeminal,  537;  trochlear, 
536 ;  vagus,  98,  182,  538  ;  vasocon- 
strictor, 93  ;  vaso-dilator,  96;  vaso- 
motor, 90,  93,  231 ;  vestibular,  422, 
542.    • 

Nerve-cell,  476,  481,  491,  516,  531 ; 
figure  of,  482,  492,  494;  general 
function  of,  483 ;  of  retina,  444. 

Nerve-centres,  cardio-inhibitory,  101, 
538  ;  diabetic,  539  ;  in  medulla  ob- 
longata, 511,  538  ;  motor,549;  res- 
piratory, 180,  538  ;  secretory,  538  ; 
sensory,  549 ;  of  speech,  549 ;  in 
spinal  cord,  510;  of  swallowing, 
538  ;  vaso-motor,  94,  514,  538. 

Nerve-fibres,  476,  484  ;  degeneration 
of,  482;  medullated,  489;  medul- 
lated,  figure  of,  486;  motor,  320; 
motor,  ending  of,  487 ;  non-rae- 
dullated,  489,  517;  origin  0^482; 
of  retina,  446 ;  sensory,  ending  of, 
489;  sympathetic,  489. 

Nerve-impulse,  320,  368,  499,  501 ; 
rate  of,  503,  560. 

Nervous  system,  475  ;  cerebro-spinal, 
475 ;  coordinating  action  of,  25 ; 
diagram  of  structure  of,  552;  and 
education,  547;    sympathetic,  514. 

Nervous  tissue,  structural  elements 
of,  481. 

Neuraxis,  481,  485,  487. 

Neuraxon,  485. 

Neurilemma,  485. 

Neuroglia,  490,  534. 

Neuroglia-cells,  figure  of,  491. 

Neuron,  476,  481,  491,  516,  531; 
figure  of,  482,  492,  494;  general 
function  of,  483;  see  also  Nerve- 
cell. 

Nitrogen,  in  blood,  149;  daily  elimi- 
nation of,  294. 

Nitrogen  starvation,  297. 

Nodes  of  nerve-fibre,  485. 

Noises,  415. 

Nose,  387 ;  figure  of,  156, 253,388, 390. 


Nucleus  of  cell,  32 ;  function  of,  482. 
Nutrition,  291;  statistics  of,  292,  557. 

Odontoid  process,  349. 
CEsophagus,  8,  254  ;  function  of,  261 ; 

structure  of,  262. 
Olfactory  epithelium,  389;  figure  of, 

391- 

Olfactory  lobes,  535 ;  nerves,  535. 

Optic  chiasma,  535  ;  diagram  of,  537. 

Optic  delusions,  466 ;  nerves,  423, 
535;  thalami,  526;  tracts,  535. 

Ora  serrata,  428. 

Orbit  of  eye,  423. 

Organ  of  Corti,  402;  function  of,  417. 

Organs,  in  the  abdomen,  8;  alimen- 
tary, 21 ;  circulatory,  22;  excretory, 
23,  194;  respiratory,  23,  154;  sen- 
sory, 20,  367;  in  the  thorax,  8; 
urinary,  201. 

Origin  of  muscle,  326. 

Os  orbiculare,  408. 

Osmosis,  273. 

Ossa  innominata,  17. 

Ossicles,  auditory,  407,  412. 

Ossification,  centres  of,  336. 

Osteoblasts,  337. 

Otoliths,  399  ;  function  of,  422. 

Overtones,  416. 

Ovum,  32;  segmentation  of,  32. 

Oxidation  in  the  body,  24. 

Oxygen,  in  blood,  132,  133,  149,  152; 
effect  of,  on  red  corpuscles,  151 ; 
in  excretions,  4;  in  muscle,  323; 
starvation,  186;  taken  in  by  the 
lungs,  24. 

Pacinian   corpuscle,  376;    figure    of, 

377- 
Pain,  sensation  of,  380. 
Palate,  hard,   253;    hard,  figure  of, 

384 ;  soft,  254. 
Paleness,  92. 

Palpitation  of  the  heart,  103. 
Pancreas,  8;  figure  of,  245 ;  figure  of 

cells  of,  283;    secretion  by,   282; 

structure  of,  282. 


INDEX 


573 


Pancreatic  juice,  283,  286. 
Papillae,  circumvallate,  384;  dermal, 
215,  374;  filiform,  384;  fungiform, 

384- 
Papillary  muscles,  69;  action  of,  78. 
Paralysis,  506,  539. 
Parotid  gland,  262 ;  figure  of  cells  of, 

265. 
Patella,  17,  344. 
Pelvis,  17;  figure  of,  15;  of  kidney, 

203. 
Pepsin,  271. 
Peptone,  272. 
Pericardial  fluid,  67. 
Pericardium,  67. 
Perichondrium,  42,  338. 
Perilymph,  393. 
Perimysium,  311,  312. 
Perineurium,  484,  485. 
Periosteum,  328,  338. 
Peristaltic  action,  262,  325. 
Peritoneum,  201,  277. 
Perspiration,  sensible  and  insensible, 

224. 
Petrous  bone,  393. 
Peyer's  patches,  280. 
Phalanges,  7. 
Pharynx,  8,  154. 
Phosphene,  451. 
Phrenic  nerves,  181. 
Physiology,  scope  of  human,  2. 
Pia  mater,  477. 
Pigment-cells,  of  retina,  443;    figure 

of,  426,  448. 
Pillars,  of  diaphragm,  166 ;  of  fauces, 

254- 
Pineal  gland,  523. 
Pinna,  394,  410. 
Pituitary  body,  523. 
Pivot  joints,  348. 
Plasma,  120;  proteids  of,  134 ;  solids 

in,  I3S- 

"  Playing  at  sight,"  546. 
Pleura,  160. 

Plexuses,    choroid,   529;    of  sympa- 
thetic nervous  system,  515. 
Pneumogastric  nerve,  98,  182,  538. 


Polarised  light,  affected  by  striated 
muscle,  316. 

Pons  Vaiolii,  519,  522. 

Portal  vein,  65,  235. 

Pressure,  arterial,  83;  sensations  of, 
378 ;    spots,  378. 

Primitive  sheath,  485. 

Pronation  of  arm,  350. 

Proteids,  4,  134;  absorption  of,  289, 
290;  in  colourless  corpuscles,  129; 
digestion  of,  271,  283,  287  ;  as  food, 
250,  298,  300;  heat-equivalent  of, 
306 ;  in  lymph,  143  ;  non-diffusive, 
146 ;  in  plasma,  134 ;  tests  for,  135. 

Protoplasm,  32;  of  colourless  cor- 
puscles, 127. 

Protoplasmic  processes  of  neuron, 
481. 

Ptyalin,  265. 

Pubis,  17. 

Pulmonary  artery,  63. 

Pulmonary  capillaries,  153. 

Pulmonary  veins,  63. 

Pulp  cavity,  255. 

Pulse,  85 ;  velocity  of,  86 ;  venous, 
190. 

Punctum  lacrimale,  441. 

Pupil  of  the  eye,  426,  432. 

Purkinje,  cells  of,  531. 

Purkinje's  figures,  453. 

Pylorus,  269. 

Pyramids,  decussation  of,  539. 

Racemose  glands,  199. 

Radius,  351. 

Rage,  92. 

Reading  aloud,  nervous  mechanism 

of.  545- 

Receptacle  of  the  chyle,  112. 

Reflex  action,  368,  507,  545 ;  artificial, 
546;  of  brain,  545;  diagram  of 
paths  of,  369;  movement  the  re- 
sult of,  367;  natural,  546. 

Reissner,  membrane  of,  402. 

Rennin,  271,  273. 

Residual  air,  172. 

Residual  pouch,  258. 


574 


INDEX 


Respiration,  148;  abdominal,  169; 
amount  of  air  needed  for,  192, 
559;  costal,  168;  diaphragmatic, 
168;  effect  of,  on  circulation,  188  ; 
essential  of,  154;  internal,  153; 
mechanism  of,  162;  movements 
of,  162;  nature  of,  152;  nervous 
mechanism  of,  179 ;  organs  of,  23, 
154;  rate  of,  171,  558;  statistics  of, 
558  ;  of  the  tissues,  153. 

Respiratory  capacity,  172. 

Respiratory  centre,  180,  538;  dia- 
gram of,  183;  influence  of  blood- 
supply  on,  184. 

Respiratory  organs,  23,  154. 

Retiform  tissue,  53,  116. 

Retina,  423,  427, 441 ;  diagram  show- 
ing circulation  in,  444;  diagram 
showing  formation  of  image  on, 
433;  inversion  of  image  on,  433, 
466 ;  section  of,  445,  446 ;  structure 
of,  441. 

Ribs,  15,  162. 

Rigor  mortis,  318. 

Rods  and  cones  of  retina,  443,  452. 

Rods  of  Corti,  404. 

Rolando,  fissure  of,  525,  547. 

Rotation  of  limb,  353. 

Running,  mechanics  of,  356. 

Saccule,  396;  functions  of,  421,  542. 

Sacculi,  279. 

Sacrum,  15. 

Saliva,  263,  265 ;  action  of,  267 ;  se- 
cretion of,  265. 

Salivary  glands,  262. 

Salts  as  food,  250,  251,  301. 

Saponification,  284. 

Sarcolactic  acid,  318,  319,  322. 

Sarcolemma,  316,  485. 

Scalae  of  ear,  400,  401. 

Scaleni  muscles,  165. 

Scapula,  17. 

Schwann,  sheath  of,  485 ;  white  sub- 
stance of,  485. 

Sclerotic,  424. 

Sebaceous  glands,  223. 


Secreting  cells,  199,  207,  219,  238,  263, 
271,  282;  figure  of,  208,  264,  265, 
271,  283. 

Secretion,  in  general,  199;  internal, 
201,  244,  246,  248;  three  senses  of 
word,  199. 

Semicircular  canals,  395;  functions 
of,  421,  541. 

Semilunar  fold  of  eye,  441. 

Semilunar  valves,  72. 

Sensations,  auditory,  414;  coales- 
cence of,  459 ;  of  colour,  453 ;  and 
consciousness,  369;  of  light,  448, 
451;  of  movement,  383;  muscular, 
382  ;  of  pressure,  378  ;  referred  to 
objects,  371 ;  and  sensory  organs, 
367;  simple  or  composite,  459; 
subjective,  370,  463  ;  tactile,  locali- 
sation of,  381 ;  of  temperature,  378. 

Sense,  of  contact,  382 ;  of  hearing,  392; 
of  movement,  383  ;  muscular,  382  ; 
of  pain,  380;  of  pressure,  378  ;  of 
taste,  383  ;  of  temperature,  378  ;  of 
touch,  373  ;  of  smell,  387. 

Senses,  special,  370. 

Sense-organs,  20,  370;  accessory  part 
of,  372;  essential  part  of,  372;  gen- 
eral plan  of,  371 ;  and  motor  or- 
gans, diagram  showing  relation  of, 

553- 
Sense-organules,  372. 
Sensorium,  auditory,  414 ;  visual,  448, 

452. 
Septum  lucidum,  524. 
Septum,  nasal,  387. 
Serous  membranes,  67. 
Serum,  of  blood,  137 ;  of  muscle,  318. 
Serum-albumin,  136,  143. 
Serum-globulin,  136,  143. 
Sexes,  differences  of  respiration  in, 

169;  differences  of  voice  in,  363. 
Sighing,  170. 
Sight,  organ  of,  423. 
Single  vision  with  two  eyes,  472. 
Size,  judgment  of,  468. 
Skeleton,  12. 
Skin,  diagram  of  structure  of,  216; 


INDEX 


575 


as  excretory  organ,  23,  223;  excre- 
tory products  of,  23,  223,  559 ;  func- 
tions of,  23,  223,  373 ;  and  kidneys, 
212;  lungs,  and  kidneys,  226;  as 
sense-organ,  373;  statistics  of  ex- 
cretion by,  559 ;    structure  of,  10, 

215- 

Skull,  6;  cavity  of,  8;  side  view  of, 
14. 

Smell,  sense  of,  387 ;  and  taste,  386. 

Sneezing,  170. 

Soaps,  284,  287. 

Solidity,  judgment  of,  473. 

Sounds,  of  heart,  82;  localisation  of 
419;  musical,  415. 

Space,  subarachnoid,  477,  529;  sub- 
dural, 477. 

Spectacles,  use  of,  437. 

Spectra,  auditory,  463 ;  ocular,  464. 

Speech,  363 ;  centre  of,  549. 

Sphincter  muscle,  203. 

Spinal  bulb,  517,  519,  521 ;  function 
of,  538. 

Spinal  canal,  7. 

Spinal  column,  figure  of,  13. 

Spinal  cord,  7,  475  ;  anatomy  of,  477  ; 
figure  of,  476,  479,  480,  493 ;  func- 
tions of,  506 ;  microscopic  structure 
of,  490;  paths  of  conduction  in, 
511;  reflex  action  through,  506; 
structure  of,  at  various  levels,  492. 

Spinal  ganglia,  479 ;  structure  of,  494. 

Spinal  nerves,  478 ;  diagram  of  dis- 
tribution of,  516  ;  diagram  of  roots 
of,  497 ;  function  of  roots  of,  495. 

Spiral  lamina,  399. 

Spleen,  8,  244;  figure  of,  245. 

Spontaneous  actions,  367. 

Stapedius,  410,420. 

Stapes,  407,  412. 

Stationary  air,  173. 

Steapsin,  284. 

Stereoscope,  473. 

Sternum,  15. 

Stimulus,  12,  368. 

Stomach,  figure  of,  269;  structure  of, 
368. 


Stroma,  124. 

Subarachnoid  space,  477,  529. 

Subdural  space,  477. 

Sublingual  glands,  262. 

Submaxillary  glands,  262;  figure  of 
cells  of,  264. 

Succus  entericus,  282. 

Sulci,  547. 

Supination  of  arm,  350. 

Supplemental  air,  172. 

Suprarenal  bodies,  247. 

Suspensory  ligament,  425. 

Swallowing,  261. 

Sweat,  composition  of,  223  ;  quantity 
of,  224;  secretion  of,  224. 

Sweat-glands,  217 ;  and  body  tem- 
perature, 230 ;   figure  of,  218. 

Sweet-bread,  8. 

Sylvius,  aqueduct  of,  523 ;    fissure  of, 

525.  547- 
Sympathetic  ganglia,  7,  475,  514. 
Sympathetic  nerve,  effect  of  cutting, 

92. 
Sympathetic  nervous  system,  7,  475, 

5*4- 

Synovia,  17. 
Synovial  fluid,  346. 
Synovial  membrane,  17,  346. 
Synovial  sheaths,  327. 
Systole  of  heart,  75. 

Tactile  corpuscle,  217,    374;    figure 

of,  374- 

Taste,  sense  of,  383 ;  and  smell,  386. 

Taste-buds,  385. 

Taurocholic  acid,  240. 

Tears,  441. 

Teeth,  253,  254;  alveolus  of,  255; 
bicuspid,  255 ;  canine,  255  ;  crown 
of,  254  ;  development  of,  257  ;  fangs 
of,  255  ;  figure  showing  structure  of, 
256,  259;  incisor,  235;  milk,  258, 
260;  molar,  255;  permanent,  258  ; 
pulp  of,  255,  257;  structure  of, 
255;  wisdom,  260. 

Temperature  of  body,  227 ;  in  fever, 
233  ;   regulation  of,  94,  228,  229, 232, 


576 


INDEX 


Temperature,  effect  of,  on  clotting  of 
blood,  138 ;  sensations  of,  378. 

Tendon,  52,  312,  327. 

Tensor  typani,  410,  420. 

Terror,  92. 

Tetanus,  323. 

Thalami,  optic,  526. 

Thaumatrope,  471. 

Thoracic  duct,  111 ;  effect  of  respira- 
tion on,  191 ;  figure  of,  113. 

Thorax,  160  ;  changes  in  size  of,  166 ; 
figure  of,  157,  161,  163,  171. 

Thymus  gland,  246. 

Thyroid  body,  246. 

Thyroid  cartilage,  357. 

Tidal  air,  172. 

Tissue,  in  general,  10,  34;  adenoid, 
116;  adipose,  53;  areolar,  112; 
cancellous  or  spongy,  325  ;  a  com- 
pound structure,  30;  connective,  11, 
49;  elastic,  in  the  lungs,  159,  162; 
embryonic,  31;  epithelial,  34;  glan- 
dular, 35  ;  minute  structure  of,  30  ; 
muscular,  34;  nervous,  34;  osse- 
ous, 325,  330;  renewal  of,  21; 
vascular,   35. 

Tone,   fundamental,    416;     musical, 

4i5- 

Tongue,  figure  of,  384 ;  in  speech,  365. 

Tonsils,  254. 

Tooth  sac,  258. 

Touch,  localisation  of  sensations  of, 
381 ;  organs  of,  373. 

Trachea,  155;  structure  of,  156. 

Tracts,  crossed  pyramidal,  513,539; 
crossed  pyramidal,  diagram  of, 
551;  optic,  535;  postero-median, 
512,541;  pyramidal,  552;  of  spinal 
cord,  512;  of  spinal  cord,  diagram 
of,  513,  514. 

Trapezium,  348. 

Tricuspid  valve,  69. 

Trypsin,  283. 

Tubules  of  kidney,  203,  205. 

Tympanic  membrane,  406. 

Tympanic  muscles,  409 ;  functions 
of,  420. 


Tympanum,  401,   406. 
Tyrosin,  284,  287. 

Ulna,  350. 

Unit  used  in  histological  measure 
ment,  40. 

Urea,  194,  209,  210;  heat-equivalent 
of,  306;   history  of,  213. 

Ureter,  8,  201. 

Urethra,  202. 

Uric  acid,  209. 

Urinary  organs,  201 ;  figure  of,  202. 

Urine,  208;  composition  of,  209;  dis- 
charge of,  203,511 ;  quantity  of,  210; 
secretion  of,  211. 

Uriniferous  tubules,  203,  205;  figure 
showing  course  of,  207. 

Utricle,  395;    functions  of,  421,  541. 

Uvula,  254. 

Vagus  nerve,  98,  182,  538. 

Valve  (or  valves),  cardiac,  69;  car- 
diac, action  of,  76;  cardiac,  figure 
of,  70,71,  72,  73;  ileo-cascal,  274; 
ileo-ccecal,  figure  of,  276;  semilu- 
nar, 72;  venous,  59;  ofVieussens, 

523- 

Valvulae  conniventes,  279. 

Vaso-constrictor  nerves,  93. 

Vaso-dilator  nerves,  96. 

Vaso-motor  centre,  94,  514,  538  ;  dia- 
gram of,  97. 

Vaso-motor  nerves,  90, 93, 231 ;  course 

of,  95- 

Vein  (or  veins) ,  22 ;  and  arteries,  56 ; 
coronary,  65 ;  hepatic,  65,  236 ;  in- 
terlobular, 238;  intralobular,  237; 
portal,  65,  235;  pulmonary,  63; 
structure  of,  59;  figure  showing 
structure  of,  60;  valves  of,  59; 
valves  of,  figure  of,  61. 

Velum  interpositum,  523,  529. 

Vena  cava  inferior,  63,  65. 

Vena  cava  superior,  63. 

Ventilation,  191. 

Ventricles,  of  brain,  first,  524;  ot 
brain,  second,  524;  of  brain,  third, 


INDEX 


577 


523;  of  brain,  fourth,  521 ;  of  brain, 
fifth,  524;  of  brain,  lateral,  524;  of 
brain,  lateral,  diagram  of,  524;  of 
heart,  68  ;  of  larynx,  358. 

Ventriloquism,  420,  465. 

Vermiform  appendix,  276. 

Vertebrae,  7,  15. 

v'ertebral    column,    figure    of,     13, 

354- 

Villi,  112,  280;  figure  of,  281;  struc- 
ture of,  280. 

Viscera,  figure  of,  275. 

Visual  image,  inversion  of,  433,  466; 
referred  to  an  object,  467. 

Visual  sensorium,  448,  452. 

Vital  actions,  25. 

Vital  capacity,  172. 

Vitreous  humour,  424. 

Vocal  cords,  155,  356,  357. 

Voice,  361 ;  accuracy  of,  362 ;  modu- 
lation of,  363;  production  of,  356; 
quality  of,  362 ;  range  of,  362. 

Vowels,  364. 


Walking,    mechanism    of,    355,   510, 

54°. 
Wallerian  method,  499. 
Wandering  cells,  51. 
Waste,  made  good  by  food,  249;  of 

the  tissues  poured  into  the  blood, 

193;  which  leaves  the  lungs,  175. 
Water,  absorption  of,  288 ;    in  food, 

251 ;    quantity  given  off  by  lungs, 

175 ;  camera,  431. 
White  matter,  of  brain,  480,  529;  of 

spinal  cord,  480. 
Will,  542. 
Winking,  545. 
Work,  of  body,  1,  305,  556;   unit  of, 

304 ;  and  waste,  3. 

Yellow  spot  of  eye,  442,  448. 
Young-Helmholtz   theory   of  colour 
vision,  456. 

Zoetrope,  471. 


Printed  in  the  United  States  of  America. 


nPHE  following  pages  contain  advertisements  of  a 
few  of  the  Macmillan  books  on  kindred  subjects 


A  TEXT-BOOK  OF  PHYSIOLOGY. 


By  Michael  Foster,  M.A.,  M.D.,  LL.D.,  F.R.S.,  Professor  of  Physi- 
ology in  the  University  of  Cambridge,  and  Fellow  of  Trinity  College, 
Cambridge.  Svo.  With  Illustrations.  Sixth  Edition.  Largely 
Revised. 

Part     I.    Blood;  The  Tissues  of  Movement;  The  Vascular  Mechanism.  $2.60. 

Part  II.    The  Tissues  of  Chemical  Action;  Nutrition.    $2.60.    In  the  Press. 

Part  III.    The  Central  Nervous  System.    $1.75. 

Part  IV.    The  Central  Nervous  System  (concluded) ;  The  Tissues  and  Mechan- 
isms of  Reproduction.    $2.00. 
.Part    V.     (Appendix)  The  Chemical  Basis  of  the  Animal  Body.    By  A.  Sheri- 
dan Lea,  M. A.,  Sc.D.,  F.R.S.    $1.75. 

"The  present  edition  is  more  than  largely  revised.  Much  of  it  is  rewritten,  and 
it  is  brought  quite  abreast  with  the  latest  wave  of  progress  of  physiological  science. 
A  chief  merit  of  this  work  is  its  judicial  temper,  a  strict  sifting  of  fact  from  fiction, 
the  discouragement  of  conclusions  based  on  inadequate  data,  and  small  liking  shown 
toward  fanciful  though  fascinating  hypotheses,  and  the  avowal  that  to  many  ques- 
tions, and  some  of  foremost  interest  and  moment,  no  satisfying  answers  can  yet  be 
given."  —  New  England  Medical  Journal. 

"  It  is  in  all  respects  an  ideal  text-book.  It  is  only  the  physiologist,  who  hss 
devoted  time  to  the  study  of  some  branch  of  the  great  science,  who  can  read  between 
the  lines  of  this  wonderfully  generalized  account,  and  can  see  upon  what  an  intimate 
and  extensive  knowledge  these  generalizations  are  founded.  It  is  only  the  teacher 
who  can  appreciate  the  judicial  balancing  of  evidence  and  the  power  of  presenting  the 
conclusions  in  such  clear  and  lucid  forms.  But  by  every  one  the  rare  modesty  of  the 
author  in  keeping  the  element  of  self  so  entirely  in  the  background  must  be  appreci- 
ated. Reviewing  this  volume  as  a  whole,  we  are  justified  in  saying  that  it  is  the  only 
thoroughly  good  text-book  of  physiology  in  the  English  language,  and  that  it  is 
probably  the  best  text-book  in  any  language."  — Edinburgh  Medical  Journal. 


FOSTER'S  TEXT-BOOK  OF  PHYSIOLOGY.  In  One  Volume. 
Svo.  Cloth,  $5.00.  Sheep,  $6.00.  Abridged  and  revised  from 
the  Sixth  Edition  of  the  Author's  larger  work  published  in  five 
octavo  volumes. 

This  new  edition  contains  all  the  illustrations  included  in  the  larger  work,  and  is 
published  in  one  octavo  volume  of  about  1400  pages.  It  contains  all  of  the  author's 
more  important  additions  to  the  complete  work,  and  is  like  the  sixth  edition  of  that 
copyrighted  in  this  country. 

PHYSIOLOGY  FOR  BEGINNERS.  New  Edition,  with  an  addi- 
tional Chapter  on  Alcohol  and  Food.  By  Michael  Foster  and 
L.  E.  Shore.     Now  ready.     i6mo.     Price,  75  cents. 


THE   MACMILLAN   COMPANY 

K  FIFTH  AVENUE,   NEW  YORK 


CONSTIPATION   IN   ADULTS  *AND   CHILDREN/ 

With  special  reference  to  Habitual  Constipation  and  its  most  Success 
ful  Treatment  by  the  Mechanical  Methods,  by  H.  Illoway,  M.D., 
formerly  Professor  of  the  Diseases  of  Children,  Cincinnati  College 
of  Medicine  and  Surgery;  with  many  plates  and  illustrations.  8vo. 
£4.00 ;  sheep,  $5.00. 

"  The  work  is  not  large,  but  is  replete  with  facts  which  are  of  practical  value  to 
the  practitioner  of  medicine." —  The  Canadian  Journal  of  Medicine  and  Surgery . 

ATLAS   OF  EXTERNAL  DISEASES  OF  THE  EYE 

By  A.  Maitland  Ramsay,  Ophthalmic  Surgeon,  Glasgow  Royal  Infirm- 
ary; Professor  of  Ophthalmology,  St.  Mungo's  College,  Glasgow; 
and  Lecturer  on  Eye  Diseases,  Queen  Margaret  College,  University 
of  Glasgow.  With  30  full-page  colored  plates,  and  18  full-page 
photogravures.  Sold  only  by  subscription.  4to.  Half  morocco, 
gilt  top.     #20.00. 

"  A  work  of  great  beauty.  The  illustrations  are  unrivalled,  many  of  them  master, 
pieces  in  their  kind.  The  text  gives  connected  descriptions  of  the  diseases,  supple- 
menting the  stages  and  phases  not  presented  in  the  illustrations.  It  is  prepared  with 
the  utmost  care  as  to  precision  and  comprehensiveness  of  language.  The  book  is 
written  for  the  observing  student,  describing  the  etiology,  symptomatology,  and 
pathology  of  the  diseases,  but  omitting  the  treatment.  The  whole  work,  which  in 
care  of  preparation  and  elegance  of  getting  up,  appeals,  in  contrast  with  the  book  of 
Haab,  to  a  select  class  of  readers,  is  an  ornament  to  S-otch  ophthalmology,  and  in 
particular  to  Glasgow,  the  place  from  which  emanated  the  best  '  practical  treatise  on 
the  diseases  of  the  eye'  before  the  discovery  of  the  ophthalmoscope  —  the  classical 
text-book  of  William  Mackensie."  —  H.  K.,  Archives  of  Ophthalmology ,  New 
York,  Dr.  H.  Knapf,  Editor. 

THE   FUNDUS   OCULI 

With  an  ophthalmoscopic  atlas  illustrating  its  physiological  and  patho 
logical  conditions,  by  W.  Adams  Frost,  F.R.C.S.,  Ophthalmic 
Surgeon,  St.  George's  Hospital;  Surgeon  to  the  Royal  Westmin- 
ster Ophthalmic  Hospital.  4to.  Half  morocco.  #20.00.  Sold  by 
subscription  only. 

"  A  work  which  is  a  pleasure  to  look  upon  and  an  equally  great  pleasure  to  read. 
The  book  is  a  folio  of  220  pages  of  letterpress,  illustrated  by  46  figures  in  black  and 
white,  of  exquisite  workmanship,  representing  macroscopically  and  microscopically 
those  parts  of  the  eye  which  we  see  with  the  ophthalmoscope.  Bound  up  in  the  same 
volume  are  47  large  colored  plates,  containing  107  figures,  beautifully  drawn  and 
colored,  representing  the  fundus  of  the  eye  as  seen  with  the  ophthalmoscope.  The 
discussion  of  the  different  conditions  observed  in  the  fundus  bears  evidence  of  very 
careful  observation  and  research.  The  direct,  concise,  and  lucid  manner  in  which 
the  descriptions  of  the  various  conditions  are  given  is  truly  admirable."  —  N.  Y. 
Medical  Record. 

"  We  venture  the  assertion  that  of  all  Ophthalmoscopic  Atlases  which  have  been 
produced  in  the  last  forty  years,  Mr.  Frost's  book  is  facile  princeps.  We  wish  that 
it  might  be  found  in  the  library  of  every  physician  and  surgeon." — Professor 
James  Moore  Ball,  Editor  The  State  Medical  Journal  and  Practitioner. 


THE   MACMILLAN    COMPANY 

<S6  FIFTH  AVENUE,   NEW  YORK 


■C 


\yN*mnwv.'t " 


uarfi  I**  [Jlwd™ •^m^t^uJ 


1  mU/uu^^  Myi^ti  oWm  jrtjh*^ 


COLUMBIA  UNIVERSITY 

This  bqgk  is  due  on  the  date  indicated  below,  or  at  the 
expiration  of  a  definite  period  after  the  date  of  borrowing, 
as  provided  by  the  rules  of  the  Library  or  by  special  ar- 
rangement with  the  Librarian  in  charge. 

DATE  BORROWED 

DATE  DUE 

DATE  BORROWED 

DATE  DUE 

C2B(63S)MB0 

